The number of cheap Kindle editions of Ulysses continues to grow. None of them are perfect: so far the editions generally accepted to be most free of errors (like the Gabler edition) aren't available, and the typesetting and layout are often amateurish. On the other hand, you can get a copy of a marvelous and strange book for the price of a candy bar and carry it anywhere.

• 2019 update: there is now a Penguin edition on Kindle, based on the 1960 Bodley Head edition, which would seem to be the obvious choice. However, it looks like it was poorly digitized and is riddled with errors.
• The Shmoop study version seems to be the only one that breaks the book up into 18 episodes. It adds Cliff's Notes-style commentary, for good or ill. Typesetting is pretty basic.
• The free Kindle edition is free, has the most reviews by far, but is apparently missing text (The Ballad of Joking Jesus and any other verse printed centered on the page.) Might be badly scanned. No idea what printed edition this is based on.
• Ulysses Unabridged (Illustrated) has the text the other version lacks. It's $2.99. Has only has one review, which makes me a little uneasy. No idea what printed edition this is based on. (Update: I ended up buying this one. It doesn't have chapter breaks. Despite what the one review says, it is actually illustrated. "Proteus" has a modern-day photo of Sandymount strand.)
• Project Gutenberg has a Kindle version up that lacks chapter breaks, although it keeps the three-part structure. Based on the 1922 edition.
• epubBooks also has a Kindle edition. Good luck finding out anything about it. It's probably fine. Why do I have to do this? Why isn't the answer obvious?

Recommendation: get the Project Gutenberg edition. Free, better than the "free Kindle edition", as good as the paid editions.

If you thought that was nitpicking, Google "gabler kidd".