The builtin implementation is an Adobe-derived one. My only problems with it are:
1) The version in the 1.x firmware has no zooming, making it nigh-on useless.
2) The version in the 2.x firmware has only manual zooming and panning, no intelligent defaults.
Since 99% of the time all I want to do is view half a page at a time and scroll half a page at a time, having to do that with a manual zoom selection and dragging it to pan is major overkill, especially, since turning pages is sufficiently different from panning that it tends to get it all wrong. Since I also read a lot of two-column stuff, the two-column half-page mode was a nice addition for me too.
If you want my reviewish-but-totally-non-comprehensive ideas on the DR800SG, here's why I bought it:
- The screen is exactly the right size for viewing half a letter page. It's about 7.5in tall, just an inch under the width of a letter page. In my opinion this makes it sufficient for reading technical papers, while also keeping the price within human limits. Since my primary goal was to have a device to throw all the research and technical papers I read onto, thus eliminating the need to carry around pounds of paper, this was important to me.
- Having a tablet screen is a nice little plus. First off, it's about the same physical dimensions as a Kindle 2 but the screen is much larger since there's no wasted real-estate for the rarely-used keyboard. Second, although everything is accessible without the tablet, it's sometimes just quicker with!
- I anticipated that the PDF viewer would suck for my purposes, but I also anticipated I'd be able to write a better one.
- What can I say, I'm a hacker! This is easily the most hackable eInk reader on the market. The Kindle has an SDK, but it's under the "we don't know what we're going to do with this" license, they all run Linux but this is the only one that has a totally open, actually quite standard software stack with a free and GPL SDK.
- I needed eInk. Reading long things on a backlit display is a form of torture. So even comparing it to the iPad is silly. Besides, the iPad SDK is under the "we own your sperm" license (this is the only way they can get a first-born son clause into a modern license agreement without stepping on child or slave labor laws)
So, I don't know whether it would be ideal for somebody who wasn't as much of a hacker as me, but it definitely has its niceties nonetheless.