I've finished playing through the main storyline, but haven't tackled the the TotSC content yet, but I honestly don't think my opinion will change much based on that.
In short, the iPad version is still the same Baldur's Gate that everyone knows and loves. The biggest difference from the original game is the use of the BG2 engine, which provides an improved UI and additional character options. However, that's been possible for the original game with mods for ages now and may not be anything new those who're familiar with the series. While I'm on the subject of mods, I'm not sure if there are any plans (or if it's even possible) for the use of third party mods with the iPad version. I haven't done any further investigations to determine if there are other methods of getting third party content into the game, but I suspect that unless your willing to jailbreak, you're likely to be stuck with what's been provided.
The controls work well enough, although they're not quite as precise as using a mouse. When attempting to tap on a small target (for example the cache in the tree outside of the Friendly Arm Inn) it can take several attempts before you manage to get it to register properly. On a smaller tablet such as the iPad mini, it might be even more difficult, if not outright impossible.
Another major issue is that the game tends to get bogged down when displaying certain spell effects or during large battles. The biggest offenders are persistant spell animations such as entangle. While they're on screen, the frame rate can drop to the point where the game isn't playable. The easiest way to fix the problem is to just pan the view to a side until it runs out.
During large battles, there's a tendency for the game to ignore touches or otherwise misbehave, leading to some hilarious results, such as blowing up your own party with spells or sending a vulnerable character into the fray. This isn't always an issue, and seems to be more related to certain large battles than large battles in general, which might suggest its a different kind of problem.
There are a few other occasional hiccups as well. Occasionally an NPC's script will fail to trigger, leaving the game in a bizarre limbo state where it's impossible to interact with them or anything else. This doesn't happen all that often (probably only three different times during the entire course of the game) but the only way to fix the problem is to load an earlier save.
However, this isn't too much of a problem as by the time you run into that particular bug, you'll have already learned to save and save often. Far more frequently, usually at least every two hours, the game will crash when transitioning between areas.
As I've mentioned before, playing the game on a non-retina screen or a smaller display might bring another set of problems. The text size used by the game is rather small, and the front may not be the easiest to read on a display with a lower PPI. Similarly, with a physically smaller screen, being able to precisely target touches may be problematic. I'll have to pick up the Android version when its released to try it out.
Outside of a few nagging bugs, the game is really solid and plays well. Hopefully an update will be released soon that will contain some much needed tablet optimizations, which I suspect should fix the most annoying problems (graphical slowdowns and random crashes). If you're a fan of the original and haven't yet picked it up for the PC or if you're an iPad owner looking for a game with more depth than the usual fare, it's definitely worth the $10 asking price.
I definitely need to check into adding some third party mods and how easily that can be done. I suspect that doing so on Android will be much easier, but for those who are used to spicing up the game with mods, you may want to stick with the PC version of the game.