For the electric guitar purists there is nothing like vacuum tubes. The response is continuous where digital circuitry is a stepped response. There are tons of codec algorithms that reproduce sound in different ways but the basis is always stepped. Most could never tell the difference, however.
Nothing like the sound of a true tube amp. However, the thing thing them is that they need to be cranked n order to get the effect to the fullest, try cranking a 100w Marshall JCM head with decent tower and try to play...amazing...but f'in LOUD.
Since I play in my house i have a smallish marshal 30w which is fully digital but comes with FDD (frequency dependant dampening IIRC), which like frothy said is an algorithm used to achieve a nice tube sound. TBH at normal levels I would be hard pressed to find a difference between it and a tubed one. This is why I prefer my current digital amp to a tube one, this is just my opinion based on my situation, if I was playing in a band or was in a situation where I needed something louder a tube would be the only thing on my mind.
I've never tried an amp with tubes :-/, I've got a 15W Marshall and a 300W bass amp, great sound when you feed the Marshall through the bass amp Maybe I'll pay a visit to the guitar store at the weekend though