Sparkyjay:
Good question, one that seems to be more frequently asked these days, what with the high-quality detailing of some of the newer plastic steamers.
In my case, except for two locomotives, it's the only way I can get steam for the railroad that I model--the Rio Grande standard gauge. The two non-brass Rio Grande locos that I have are the Proto2000 USRA 2-8-8-2 Rio Grande L-107 clone, and while it is a beautiful locomotive, a smooth runner and exquisitely detailed, it doesn't have near the pulling power of my Oriental Limited brass model of the same locomotive. However, it's a great 'helper' locomotive (which was basically what the prototype was used for) and looks GREAT!
The other non-brass Rio Grande loco is the L-97 4-6-6-4 which is an almost exact copy of the Union Pacific Challenger. Rio Grande was assigned these locos late in WWII, didn't like them at all and sold them as fast as they could. Mine's a very powerful Genesis loco, but unforunately, since my MR is DC, and the loco is dual-mode DC/DCC, I can't run it with any of my other locos, so it sees very limited usage on my railroad (somewhat like the use the prototypes saw on the real Rio Grande, LOL!).
However, since those are the only two non-brass Rio Grande prototypes, and the Rio Grande is my favorite railroad, it's pretty necessary that I go brass for the other prototypes. Over the years, I've collected a fairly large (around 50) brass Rio Grande prototypes. Some of them have run magnificently straight out of the box, others have had to be worked on a bit, ALL of them occasionally take some 'tinkering' if they've been run a lot (what the heck, so did the prototypes!). Just about all of them have been re-weighted or re-balanced at one time or another, and just about all of them are capable of handling almost as many cars as their various prototypes. Traction tires are non-existant on my MR, I don't even know what they look like.
I accept the detailing on brass as a matter of course. Some of my locos are much more detailed than others, but that's part of the 'growth' of the brass industry over the years--more and better detailing. Some of my older brass locos look a little 'bare' compared to my newer locos--and I've cured that with castings from Cal-Scale and PSC to a great degree, but I also use the '3-foot' rule--if it looks presentable from that distance, and runs well and pulls what it's supposed to, then I'm a Happy Camper.
It's been said that older brass was built to run and newer brass is built to 'look at', but I think that's a fallacy. Actually, most of my older brass has been re-motored and in some cases re-geared, and most of my newer brass runs very smoothly (occasionally with a little work). My newest brass locomotive, a PSC Rio Grande F-81 2-10-2 has been absolutely flawless since the day I unwrapped it and put it on the tracks. I'd match it any day of the week for control, smoothness and power against anything currently available from BLI or MTH.
So, to sum it up: Brass will get you the locos that you can't get from contemporary plastic. Not everyone that likes road-specific steam models UP, PRR or N&W.
Brass often takes more work, but is very forgiving--and simpler-- to work with. And it's one Heck of a lot easier to get into than current plastic locos if you need to do any motor or gear tuning.
Brass can be re-weighted and re-balanced quite easily. Try and find room in the current plastic boilers for additional weight--it's all taken up with electronic geegaws.
Brass mechanisms are almost indestructable. I've never had a split gear in any of my brass locos, and some of them date from the 1960's.
Negative:
Brass, because of its left-right pickup between loco and tender does not always take kindly to DCC without additional wiring. I'm DC, so this is a non-issue. But for DCC users used to all-wheel pickup, this can sometimes be a detriment to conversion.
New Brass is EXPENSIVE!!! But---you can find used brass at many hobby shops that feature a large selection, for often less than the price of a new plastic steamer. It might have to do with the economy, but a lot of older brass 'collections' are appearing on the market at very reasonable prices.
Brass takes 'tinkering'. It comes with the territory, so to speak. But brass owners like myself automatically take that into consideration. We really, for the most part, don't mind the 'tinkering', because the result is usually a very fine running, fine looking locomotive.
Most brass is built to much tighter tolerances than current plastic. A brass Articulated isn't going to go around an 18" curve, simply because brass is 'prototypically' articulated--only the front set of drivers swings. In fact, many non-articulated long wheel-based brass locos won't accept anything less than a 30" radius. Example: I have several older brass articulateds that can squeeze around a 24" radius if called upon--with a FEARSOME boiler overhang--, yet I have several 4-8-2, 4-8-4 and 2-10-2 brass non-articulateds that DEMAND at least a 30" radius. The MTH UP 4-12-2 will, I understand, traverse a 22" radius with ease. A brass version of the same model requires something closer to a 42" radius.
So in conclusion, I would say that if you're DCC, have a MR with relatively tight radii and are modeling a prototype that is fairly well represented steamwise with the current locos from BLI, MTH, Proto or Spectrum, go for it. They're all pretty nifty locos.
But if you're like me and have chosen one of the railroads in a 'niche' category where the only way you can go is brass--well, then, you go for it and adjust.
And believe me, I've never regretted the adjustment. Not one bit. You can't get this puppy in plastic, but in brass--oboy, does she do the job!

Tom 