I recently had mine done, pretty easy if you know what you are doing and takes less than an hour. I bought a service Kit off ebay for about £45 or £50 as my old piston was rusty and jamming the front wheel - Comes with all seals and grease etc and this is basically all you need unless you can find a set of cheap calipers for the same price (I wouldn't spend the £200 or so for a refurbished one) - I would advise doing both at the same time, roughly 1 hour per caliper. I think the front service kit is dearer than the rear too.
Most Calipers are the same so may be good to look at Youtube for reference if you are doing it yourself and may take about two hours per caliper - remember to bleed the brakes and top up your brake fluid after it, this too is pretty straight forward. As I earlier said, if you are going to replace the caliper at a garage or yourself, it can be a hell of a lot cheaper to service the existing one as most of the work is removing the caliper anyway.
I only serviced one side but if I get round to doing the other I will try to video the process and post a link.
I would have done a step by step guide which I will do when I get round to servicing the other Caliper.