I have two. I have a small one and a bigger one. The small one is an M-Audio Micro Track II. It fits in your pocket. It has 1/4" jack inputs, so you need an adapter to go to RCA jacks. It makes excellent recordings as high as 96/24 wave files. My other one is a Marantz PMD671. It is about as big as a book. The Marantz is the top of the food chain when it comes to digital recorders. I like it because it has a third head. So you can listen at the same time it is recording. You can even rewind it and play back a different portion of what you recorded, while it is still recording. It has RCA inputs and outputs. It has tons of features, but it is not cheap. It is what the court reporters use. I bought mine used, so I saved a bunch. The Micro Track is nice because it is small, due to the fact that it doesn't have built in mics, so it is smaller than most other portable units that have built in mics. I does come with a couple of small mics incase you want to use them.
I think all those small digital recorders pretty much sound the same as long as you use the line input. On the expensive ones, like my Marantz, you are paying for the mic preamps, and the extra features. So if you are going to get one just to record through the line inputs, I don't think it makes much difference which one you get.