There are some reviews here (admittedly old, from 2008):http://www.macworld.com/article/1135333/crosswordapps.html
I've recently used Stand Alone Inc's "Crosswords" app for my Android tablet, and I really like it. I found entering the solution perfectly comfortable.
I've also used "Crosswords" on an iPod Touch (much smaller than a tablet), and found it just too small there. But I wouldn't want to solve a crossword on a small mobile phone device; it's just too hard IMHO.
FWIW there's also the Puzzazz
app which lets you solve crosswords, among other things - I think if you search for Puzzazz in the app store on any Apple device, you should find it. I've only tried it on my iPod Touch, and again just found it really difficult to use because of the size of the device. Overall it's a nice app, though, and has crosswords by some well-known cruciverbalists.
Full disclosure: I represent World of Crosswords, and we provide puzzles both to Puzzazz and Stand Alone.