It's a very good app, bordering on greatness. But there are a few things that keep that fifth star off, for me, at least.
(All of these observations apply to the 3.5 version of the app; I haven't tried the Pathfinder side of things.)
Without going into excruciating detail (and I do tend to be verbose), play around with the app. A lot.
There are several idiosyncrasies that could trip you up, but after building a few test characters and increasing them up thru a bunch of levels (all the way to 20th, I'd recommend) you'll figure them out.
The one that is just a major head scratcher for me is when it prompts you to select a new feat. For Ranger, Rogue and Barbarian, at least (I didn't try Fighter, so it may be different, but I doubt it), the app prompts you to add a feat at 3rd level, no problem. But then it prompts you for a new feat at 5th and 7th, but not at 6th. 9th is OK, but not all of the levels prompt you to add a new feat correctly. (You can manually add feats, and really pretty much anything, in Edit mode; you have much more flexibility in Edit mode than when prompted, but the prompted screens let you do simple adds, which is often all you need.)
A trick I'd like to impart is, don't "Add Class" from the main screen when you are initially creating your characters. Instead, choose "Advance in Level", and then select the Class you want your character to start as. This sets all of the "Class" flags in the Skills list for that Class. IF YOU DON'T DO THIS, IT DOESN'T SET THESE FLAGS PROPERLY, and that means that Skills won't work as they're supposed to, which is very sad.
This will set your character at 1st level.
Then, when it's time to go up a level, choose "Advance in Level", and when it prompts you to add a level to a Class, be sure to select your current level Class entry; this is the first choice on the page, do not select the same Class from the complete list of Classes below your current Class(es). If you select the same class twice, the app won't stop you, even tho this is specifically verboten in the Player's Handbook.
Likewise, if you want to go multiclass, choose "Advance in Level" and when prompted, select the new class from the complete list of Classes. You WILL get all of the correct Class flags set for that Class' Skills if you do it this way; otherwise, you may not. Do not select "Add Class" to multiclass your character.
And, finally, if you want to create another character that's the same as (or similar to) one you already have, don't Copy an existing character. None of the Class flags in the Skills section will import over to the new character sheet if you do. You cannot fix this via the app in any way that I've tried, although there still might be a way. (Copying items from one character to another, when you're adding items to a character, does work fine, including copying over any bonuses, etc., that you set on the original character's item.)
Otherwise, it's a very robust app. Just not 100% great.
But, now that I have it on both my iPhone and my iPad, I couldn't imagine doing a D&D 3.5 Session without it!
Be sure to really play around with the features. It's very feature rich and capable, but a wee bit quirky in some places.
(OK, I ended up getting verbose. So, sue me. I hope this helps someone!)