I have been using my iPad for legal research for awhile now. I love the fact that it is much like reading a book, and just seems easier to browse larger cases and treatises than on my Macbook Pro (although I am liking some of the full screen reading options in the newer Mac apps). BNA Tax Management Portfolios work great, no issues at all. With Printopia setup on my Macbook Pro, I can even print if I need to. I do not subscribe to WestlawNext, and instead subscribe to regular Westlaw. After shutting off popups of cases in settings on Westlaw, and remembering to use two fingers to scroll on my iPad, this seemed to work great in Mobile Safari. However, recently I have had difficulty in getting the cases to display properly, and have switched to using icab mobile (with icab set to display in regular safari mode). This seems to work great.
As for apps, I really like the Blacks Law Dictionary, since it lets me search by topic under the key number system on Westlaw (I may be the only person still using key numbers....I don't know). I also have the tax code and regs on there, and Fastcase as a backup for looking up case law. There are some great financial calculators as well.