I do have some gripes with my 505. I'm trying to edit myself down here, from my original 5-page version of just a couple silly points. Here's the short version, and let me know if you have questions:
- PDF support is great, but many PDF's suffer on the 6" screen (even in landscape)
- Reflow is also great, but its effectiveness is a crapshoot
- LRF files (very common, especially in the free books section here on mobileread) have to perform a pagination process the first time you open them up or switch fonts; this can be several minutes long if the book is huge
- ePub currently doesn't do justified text (the spec was updated to support it, hopefully we'll see a firmware update soon); I'm also a little confused about how ePub handles page numbering.
- the little navigational wheel (up/down/left/right+enter) is annoying if you are navigating among a lot of items (screen updates are slow); this problem crops up with news feeds from Caliber (not too bad, though) and the contents and index sections of tech books that I bought (bad; tap...tap...tap...(repeat 10 more times)...enter!)
- shopping for ebooks online is a frustrating experience, since there is no ubiquitous format (and no device that supports it). PDF is actually the closest thing to one, but see the first two bullet-points on that.
- as of ten minutes ago, a new irritation! I just learned that Adobe Digital Editions are built from one of two publishing formats: PDF and ePub. ePubs work great on the reader, but there's no way to tell if a particular ADE book for sale is built using PDF or ePub.
- the free flip cover is good for protection, but annoying while reading (IMO); I often pop the reader out of it (easy to do). I wish it was able to bend all the way back without feeling like the spine was going to pop off (in a permanent "oh it's broke now" kind of way).
The basic stuff like controls (apart from in-page navigational ones as I mentioned above), screen quality, form-factor and comfort are superb.