In a vacuum tube, nobody can hear you scream.

[Magpie]

I don't know about other uses but for guitar amplifiers the valve sound is still the preferred sound by electric guitarists for it's warmth in the way it produces low frequency harmonics. Also because valves react differently depending upon how much energy/power you put through then they become an expressive extension of the guitar which feels more organic than solid state transistors. I've been assured that the reason for all this is because valves work in a non-linear way, which I'm assuming is the same as saying it 'feels organic' which is good enough for me. I can hear it and I know.

The main drawback to valves is the weight which is ok if you don't have to move it, but usually guitar amps see a lot of miles on the road so it becomes an issue.

Digital attempts to replicate the same sounds and the feeling of valve amps always have and probably always will fall short. And it's not that I secretly despise digital like a lot of others but in this aspect for valve guitar amps there are very good reasons for why they can't match up, mainly the response that you get back. It's the same with effect pedals, digital software has come up with amazing packages that give you 100's of effects for virtually nothing, but some people are still happier paying over £50 for just one pedal and one effect. 

All in all I'm glad I just play acoustic guitar so it don't affect me at all these days, but it used to.