I think I might be able to help you out since I have both a T series (T21) and just picked out a new R series (R51) for my sister. I personally love both of them and I now am certain that IBM makes the best laptops in the world.
I bought the T21 used on ebay for about $680 with a 900MHz p3, 40GB hard drive, 512megs of RAM, DVD drive, floppy drive, and a case, and I still think this is the best value you can get. I bought a wireless card and a micro USB hub (since the T21s only have one USB port) and now I have a great portable system. It also still had a few months of warranty left on it (although this is not usually an issue with IBMs). This is a good solution for a laptop that is not going to be your primary computer. It is by no means a gaming system or a media station, but it is perfect for emulators, flash games, word processing, internet surfing, photoshop, programming, etc. If you are buying a used laptop, make sure you get it from an individual and not a "warehouse" for refurbs, as refurbs often have dead pixels, scratches, or other defects. Contact the seller regarding dead pixels, scratches, etc.
The R series is definitely a better value, especially if you are buying new. The only major difference is the size (which is not that big of a difference, but it definitely feels more like a desktop notebook rather than a portable solution). So if the size difference is not important and you want a DTR, I think this is the way to go over a new T41/T42. Some of these have firewire, some do not. The one we got does not (not needed for her). If you decide to get the R51, I would highly recommend re-installing windows XP pro or un-installing the IBM software, since it takes FOREVER to finish booting up after starting all of IBM's services and everthing (much longer than my T21). Also, RAM is important, but you can purchase an extra 256megs of ram from Crucial (or whoever) for much cheaper than upgrading through IBM. I agree that an external DVD burner would be much better than internal, but I would be inclined to splurge on the CD-burner/DVD combo drive for travel when you don't want to lug around that DVD burner.
To me, both laptops seem to be of the same high quality materials (no keyboard compares). When you get a new one, you get the nice bonus of stuff like the hard drive shock protection, 2 USB ports, better graphics, built in wireless, both trackpoint and touchpad, full warranty, etc. Although the R51 is 1.5 GHz, I cannot notice a difference in speed between the two. I cannot notice a difference in quality of construction either.
Hope this helps! Sorry if it is too long
edit: The T21 does get pretty warm if you are sitting in bed or on the couch, but it is not unbearable. I haven't tried it with the R51.