I've been twice, mainly because the first time we tried to cram too much in and had to skip a fair bit, there's still plenty I haven't seen and a third trip is in the planning. Unless you have a lot of time, consider only doing some of the islands bu doing them better. The West and East roads of Harris are both worth doing, it'd be a shame to have to choose. The out and back roads to Uig, Hushinish and Rhenigidale are all great riding. Those three alone would be a good weeks tour, a day riding between and a day exploring each area. It's just the way I'd do it...
I'd agree with most of what's already been said, a couple of things to add; The Gatliff Hostels are great, for a couple of quid you can also turn up and use the facilities even if you're not staying, though the one on Berneray is worth staying at for it's magical location. Not very well publicised, you can take bikes on most of the islands buses, handy if you want to get somewhere and the weather is against you. There's a fairly new place to stay in South Uist, a couple of miles from the Lochboisdale ferry terminal;
http://www.uistbunkhouse.co.uk/
Advertised as a bunkhouse, out private room was hotel quality at hostel prices and nice people running it.
I'm sure you'll have a great time, here's a few photos;
http://www.derbyctc.org.uk/pictures/oht/