I've owned the Tascam for a few weeks now. It's my first proper field recorder, and I bought it for similar reasons as why you're interested in it - financially it's a very attractive proposition.
I did a lot of research before I bought it, and found some data sheets that indicated that the preamps are very quiet, pretty close to Sound Devices 722 in fact. Have a look here and here. I also read comments from people on the Yahoo Nature Recordists group, and most were positive. Some people did encounter some issues, mainly with SD cards behaving strangely. I personally did not have this problem so far.
My personal experiences with it are very positive so far. I like the unit a lot. The sound quality of the preamps seems good to my ears. The operation is straightforward, a few quirks, but nothing that bothers me. I do have a few things that I need to figure out though - I have a clicking noise on preamps 5&6 (at about -60dB) when they're switched to the mic level. I need to work out if this is can be fixed by installing the latest firmware update - stupidly enough I haven't done this yet. Otherwise I may have a faulty unit, it seems - but it's not a fault that stops me from using it, although I will exchange it for a new unit if the problem persists.
Another thing I've found is that it may be fairly sensitive to interference, perhaps the shielding of the preamps isn't as good as what a Sound Devices might be, but I am not entirely sure of this yet either - need to do more tests, etc.
All in all, a great device. I can record 6 analogue channels simultaneously in the field and it cost me £700 only (found a good deal). With the 2 additional digital inputs I could do 8 track recordings even... The self noise of the preamps is very low, so I can use it with the quietest microphones available. The metering is a bit iffy, but so far this hasn't been an issue for me. The headphone amp is pretty loud, but noisy. As long as you remind yourself that all that noise doesn't print to the recording, you can live with it. It's small, very portable. Light-weight also.
Build-quality - well, it's a bit plasticky perhaps, but if you keep it in a good bag/case then that's fine. All in all, this is of course a very, very cheap unit for the amount of inputs you get and the quality of the preamps. So yes, the slightly plastic feel of the unit is one of the trade-offs. Also, the built in limiter and lo cut are useless (not that I want to use either of those in the field anyway) - they're useless cause they're in the digital domain, so yeah pretty pointless really.
Battery life - get good batteries and a proper charger, and you'll be OK. A friend of mine owns the Tascam as well and he used it last weekends for several hours in a row without the batteries failing him, I think he was using 2 or 3 inputs with phantom power. I haven't done very hardcore tests for the battery life yet, so far I haven't encountered any issues. Swapping the batteries goes easily as well anyway, but yeah that's not necessarily what you want to hear of course...
I considered the Fostex FR-2LE and Marantz PMD661 as well, because they also have great preamps. However these are stereo units. I figured I rather spend a few hundred pounds more and be able to record 6 channel or more. Clearly, if you want to do more than stereo channel recordings, the only option out there is this Tascam, or big shitloads of money for a Sound Devices, a Deva, and whatnot...
Feel free to fire me any questions if you want know more. If it wasn't clear, I'd definitely recommend this device :)
It now also has a M/S matrix, since the latest firmware update.