Just wondering, been looking at these guys for awhile. I hear everything from "Best" to "Worst" Marshalls ever. But Refused and Rise Against are among the bands that use them, so they can't really suck entirely...?
Any advice? I know there are a bunch of differant ones, I'm looking at the 4100.
JCM900s are pro-quality amps. They're more modern sounding than 800s, and generally don't have the fizzieness of the 2000s.
All of them are good if they work for you and provide the sounds you want.

I am not a big fan of the jcm 900. It sounds good, but it just isn't for me.
My Gear
-Gibson Les Paul Tribute (Bare Knuckle Nailbombs)
-Ibanez "lawsuit" Les Paul (Seymour Duncan Pegasus and Sentinent)
-Ibanez S470 (Dimarzio D-sonic and Humbucker from Hell)
-PRS SE Custom (Guitarforce Black Diamond and Lord of the Blues)

Marshall TSL100
EVH 5150III EL-34 50w
Marshall 1960a cab

Dunlop 535q wah
Boss Super Chorus
Bogner Uberschall
Ibanez DE7 Delay
Electro-Harmonix Power Booster
Fender PT100 Pedal Tuner

Ernie Ball Skinny Top/Heavy Bottom 10-52