Pubmed on tap:
Pubmed on tap has probably been around the longest, and is currently at version 2.5.4. As such it is a rock-solid contender, with all basic features on-board. The interface uses only standard iPhone interface elements and is very clear. Only the abstract view is a bit disappointing, it uses a font size that is just a bit too large (non-adjustable) and it also makes use of justification.
It is a universal app and the iPad interface is similarly simple, but effective. Like Pubmed Clip, it can email citations in RIS format, however switching between plain text and RIS format for email requires a dive in the settings tab. For users that have not gone completely digital yet, the Loansome Doc service is integrated, allowing users to order full-text copies of articles from their local medical library. One downside was that EZproxy support was a bit flaky, it did not seem to be consistently applied.