So Squidbait, what are your thoughts on the various makes?
Well, outside of brass, I think that Proto 2000 has probably had the most consistent high quality. Their 0-8-0 and 0-6-0s are both excellent models in terms of detail and performance. My only gripe with the 0-8-0 is that earlier models didn't pick up power from the tender, while all the 0-6-0's do. The Berkshire and 2-10-2 and the Y6 are all good-looking models too, but I've only run the Berkshire, and it's as smooth as the switchers.
Spectrum has very well detailled models, but their quality has been less consistent than Proto 2000. Their first offering, the Pensy K4, was great for its' time, and was for the most part a good runner, but there were some clunkers in there too. Next I believe was the 2-8-0, and it was great, but the follow-up. the 4-8-2, had some serious problems (as did early Spectrum diesels). I have found that it's not so much specific models that are poor runners (although I have yet to see a 4-6-0 that doesn't have a small hitch or limp to it, or a 2-10-0 that doesn't have a small bind) but that there's more variation in quality within each model. I've seen several 2-10-2's that ran as smoothly as Protos, and one that was a total dog. The Mallet is a really nice runner, as is the 0-6-0 saddle tanker.
BLI's regular steamers are very well detailled, and of the ones I have run (Hudson, Mohawk, 2-8-2, 4-6-2) have been very smooth and strong pullers. The Blue Line locos are probably some of the best deals in sound-equipped steam you can get... just not as detailled.
The Athearn Genesis 4-6-2 and 2-8-2 were both very good locos, average detail on very good chassis. I've only seen one that was a poor runner, and I think it must have been dropped, because it was brought in for repair, and had a click-jump in the drive.
I haven't seen too much of Rivarrossi's latest offerings, other than the Hudson. Their older models were the best you could get in plastic steam for a long time, but the early ones had a big 3-pole motor that cogged badly, and later ones had a 5-pole motor that was a bit smoother. Detail was mostly molded on, but decent. Huge flanges meant lots of work turning them down/replacing wheelsets, but with a bit of work these locos became very good. The Hudson is one of the smoothest-running plastic steamers I've come across. Perhaps it's just because it's got those big drivers, but it just seems to glide across the track. Very quiet, too.
Mehano/IHC has some good value for less-detailled models. The mechanisms are generally sound, the newer ones have flywheel-equipped motors, and they run fairly smoothly. The latest one I have is their 2-10-2, and it is as smooth and quiet as any Proto 2000. I've got a few of the USRA 2-8-2's and 4-6-2's from earlier runs, and they're OK, but certainly not up to current standards. They are, however, easily re-motored, and become good runners with a little work and tweaking. Their mogul is also quite nice, and they have the few old-tyme 4-4-0's still on the market.
The Roundhouse 2-6-0 and 2-8-0 never fail to impress me with how well they run. They don't have the detail of a Proto or Spectrum, but they're smooth and quiet. Older Roundhouse could become good runners with some tweaking, especially the kit locos... but detail was definitely optional, and valve gear was pretty rudimentary.
I don't have much experience with the latest Mantua releases. Their 0-6-0, for what it is, is a decent little loco, but nowhere near the level of detail of the other manufacturers, and in comparision to the Chinese steam, the drive is a little coarse. Still, for $50, what do you want? Their older steam required quite a bit of tweaking to work well, but once tweaked, pulled like crazy if it was of the metal-boilered generation.
Hmmm.... who's left? I think that covers the main players in the steam game these days... if I think of anyone else, I'll post an addendum.