I second terry2foot comments-some do take things to the extreme, and that's fine.  Give others the freedom to choose what suites them and not pass judgement...

The metal wheel sets definitely improve the 'roll' of the freight cars (as jward mentioned, they also don't attract the dirt as easily as plastic).  I use the Proto 2000 wheel sets-get 'em from Walthers when they are on sale.  Their are many quality wheel makers out there though.  Get yourself a NMRA standards gage for rp25 and I recommend trying to stay with one wheel set manufacturer, once you've decided.

I've used several, usually based on pricing at the time I'm buying -
www.walthers.com (good sales sometimes and free shipping on orders over $150)
www.tonystrains.com (DCC systems and decoders)
www.trainworld.com (good prices)

I haven't bought from the ebay sites listed, but will add them to my list of vendors I check.  I also do product searches (usually on the walthers part number because so many hobby stores use the walthers numbers) on google - every once in a while I'll come across a really good deal at a local hobby shop somewhere.

Haven't had bad service from any of these, so far (knock on wood).