Ok, I have had gel varnish a couple of times. If you are going to have it done, check with the salon first on how they take it off. They should soak it off with acetone soaked cotton wool pads, and then lift/scrape it off, I think with a wooden stick. When soaked, it should lift off, but if they talk about filing then don't go there.
When I had it done, it stayed on well for nearly 3 weeks - got fed up of the colour in the end, and you could see where my nails had grown. I found my nails were nice and strong afterwards as they had been protected from the usual wear and tear by the polish.
They slightly run a file across your nails before the varnish goes on, but not too much and thats just to rough the surface a bit to make the varnish stay on better.
I wouldn't fancy having it done all the time, but its quite good before a holiday.