I purchased MacDive a little while ago, and now that I am really starting using it, I have a bunch of questions/remarks. Please note that I tried to read the wiki/FAQ and ML, but some things I could not solve this way.
First, a note on my config: Mares Nemo wide with 3.0 firmware (the 3 gas one) and Mac OS X Lion.
1) SAC and merged dives
I am currently in training for a CMAS 2*, and that implies many dives with surface time (assisted ascents drills). Most of the time, we chat a little after surfacing, hence my computer registers separate dives, that I then merge under MacDive.
Is MacDive considering that I am using air on the surface or not ? My calculated SAC (using a tank gear item and start/end air pressure) seems a bit off in those cases (too low), this is why I am asking.
2) Certification field
First, I would suggest you add CMAS as a certification agency in the default values. I am from France, and our certs here are double-sided FFESSM(local diving federation)/CMAS. That would be nice

Second, there is a chicken-and-egg issue with some of the training dives. When I did my PADI OW, I had no other cert. But MacDive does not let me leave the certification field empty. What am I supposed to put in that field ? My OW cert that I did not have (yet) at the time of these dives ? Also, I find the whole concept of a certification associated with a dive a bit awkward. If I am diving on my own (or in the aforementioned case), I
should be able to leave the field blank. If I am diving in, say, a PADI structure on a 35m night-dive, should I attach my PADI Deep Diver cert, or my night-dive cert ? What if I am doing this in a drysuit ? etc. One thing I know is that I should be able to link in at least my AOW cert to the dive, but this is not enough on its own to justify the depth/night/etc.
One suggestion here would be to differenciate the certs (OW, AOW, Rescue, DM) from the specialties (dry suit, night, etc.). This would also work with other certification agencies than PADI (though for instance CMAS does not have specialties for depth, which are part of the certs themselves only, but has dry suit, etc. as separate specialties).
3) Gear
I would love to have generic gear items, aka the ones I rent/borrow from a shop when I go diving, in addition to my own gear. Of course, I can just add the rental gear, but it really is not clean to have the purchase date filled in that has no meaning, and all the other fields.
Second, regarding purchase price, I have some items bought as a pack, but I want to split it in my gear database. This means I have to set the price to 0 for some items, and add a comment explaining why/the pack contents. To solve this, I suggest a "Bought as pack" checkbox, that would enable a pack name. If checked, all items linked to that pack would have the same price and purchase date, and the contents of the pack would simply be the list of gear items linked to that pack name.
4) Columns preferences
I noticed that the columns sizes are not saved accross sessions. This really is annoying, as I have to resize everytime to get a clean view of my dives. This is clearly a bug (and I mean a bug, not a "missing feature", as this behaves opposite to what is naturally expected and is really annoying).
Also, when trying to edit directly a field in column view, the behavior is strange. For the same field, sometimes double click will pop the dive edition window, sometimes it will allow in-place editing. At first I thought it depend on the double-click speed, but after trying many things it does not. Sometimes, on a selected line, a single click is enough to edit in place, sometimes not. In fact it apparently waits a long time for a second click (triggering double click behavior -> popping the window) and after that delay, if no second click has occured, allows in-place editing. I think either the double click delay is too long or there is something fishy under the hood.
When editing a field in place, ESC should cancel the edit and revert to the previous value. It does neither.
5) Optional graphs
With my computer, I have none of the optional graphs (SAC, temp, NDT, Air). This is fine, but having all the buttons displayed anyway with a default empty Temp curve from -1 to +1 degs is both ugly and eats up screen real estate. Why not simply hide the unavailable buttons, and the whole sub panel when relevant ? That would be a much cleaner interface. When I show MacDive to friends, invariably I get the "mmmh, that seems broken, look at that buggy temp graph there" comment.
6) Bugged merged graphs
All the merged dives I have display buggy graphs: On the time axis, the graph goes beyond the right-side vertical bar that is supposed to be the graph boundary, and the depth there is wrong too. See this:

7) Documentation
The semantics of the various fields is messy. For 25 bucks, I was expecting doc and features better than open-source counterparts. This is not the case. The only doc I could find about this was on the wiki, documenting just 4 fields!! At least add predefined reasonable defaults that would be self-explanatory.
8) Save/autosave
Bad semantics. There is a save menu option, a restore from save, and apparently some sort of autosave. How am I supposed to know when autosave occurs ? What about providing a save as option to allow for more flexible experimentation (notably on merging dives, which you can undo right after the fact, but not "unmerge" at a later time. With the merged dives questions above, this is a killer one.)
9) Missing features on graphs
I noticed that you do have the code to compute ascent/descent speeds, as you display them for free dives. But I found no way on getting those VERY important metrics for my Air dives. Have I missed something there or is that not implemented ?
Also, I miss a lot the decompression stop data on the graphs. I know that the data exists somewhere, as the Mares software is capable of displaying those in a different color on the graphs. But even without going to specifics of each manufacturer's computer data format, the possibility to select and annotate/change color for parts of the graph would be a must.
10) Autocompletion in dive master field (dive inspector)
My dive master usually have both a first and last name. Also, there is only one divemaster per field. Why does the autocompletion suggestions here only gives me one at a time instead of the full name ? The completions list should be built with the whole strongs from existing fields, not space-separated substrings. This is a bug (probably also present for other fields I did not double-check/used).
11) Bottom time and merged dives
This also includes the surface time between merged dives, hence being totally different from what I have in my paper stamped logs. Why ? That should substract the total time spent on surface!
12) Synthesis mode (click on the Sigma icon).
I have two dives out of 15 dives with a SAC: dive 13 and dive 14, SACs 12.92 and 22.42 respectively. The Sigma icon, for average SAC rate displays an empty graph, with just the lowest SAC of the two as max SAC axis value, and that's it:

13) Shuffle preview mode
Unsure how you call this one. It is the first icon on the left of list and Sigma ones.
When clicking on it, I just get a black rectangle in the graph area, and my dives list below, nothing else

Conclusion)
Well, I have some other points to report, but I got tired of going back to MacDive, retesting, rechecking if I missed something, describing the problem, etc. For 25 bucks, I have mixed feelings about this software. But I like the general layout of it, the fact that is is a Mac native app, so I still have hope. Hence the time I spent testing and reporting the above. I just hope that you will be reactive and give good answers to my questions/remarks, which would mean I'll keep using MacDive. Also I intend to publish a review of a couple of DiveLog software on a couple of divers forum I participate in, so in order to be fair, I'll wait for a complete answer from you before getting on it.
Best Regards,
Jerome Martin