Mikemann's right, tubescreamers and overdrive pedals modelled after the TS dont emulate tube tone, they're meant to drive tube amps harder and work the tubes harder so that they saturate and give you a sweet overdriven tone.
I'd be careful of the current marketing trend among big name pedal manufacturers that promise 'tube tone' in pedals by adding a 12AX7. Its commonly known that the 'tube tone' that is prized from tube amps is caused by tube saturation of power tubes (6L6s, EL34s) in the power section. What most tube pedals and hybrid amps (like the marshall AVT series) offer is basically a tube preamp section. That being said, i've personally made the switch from solid state to hybrid and then to tube. I have to say that the AVT series is as good as the reviews say it is. Generally hybrid circuits nowadays are getting closer to the real thing, although tube purists would disagree.
Pedal design incorporating 'tube' technology is also improving, i've used the Tonebone which also incorporate preamp tubes in its design. It also gets you close to a marshall in a box, but then again most people get best results when they use such a pedal through a clean tube amp. Although the pedal will probably make your solid state amp sound better, but it wont make it sound like a good tube amp. The same applied with the avt - hybrid amps will sound closer to a tube amp, but it wont sound as good as a tube amp. At low to moderate volumes it sounds very close to a tube marshall, but turn it up a little and it turns to mud, so you compromise the tone for playability at lower levels.
So id say the bottom line with 'tube pedals' is, probably will sound closer to a tube amp, but results will vary with what amp you use it with. If u're going to use it through a 15watt ss practice amp, save the money and upgrade your amp is what i'd do. Hope this helps.