I have lots of experience as I'm from Cornwall and have been to IoS many times on holiday as a child, and since for a wedding.
It has become very expensive, but it is paradise if you want to get away from it all. Once you get over the shock of the cost (at least you're not loosing out with the exchange rate though) there are lots of places to choose. St Mary's is the 'busiest' island and is where the Scillonian ferry docks and aeroplane/helicopter lands. If you don't have your sea legs, fly as the crossing can be rough.
Anyway, all the islands have their own character, but for a first timer go for St Mary's. It's gorgous, the other islands are very remote and you have to get a boat between them. I think they all have a pub, but you will need to get off the islands at some point during the holiday if you're going to explore so St Mary's is the most convenient place.
I've stayed on Bryher, Tresco, St Martins St Mary's but not St Agnes which is the smallest island.
It's such a magical place. I'm so jealous you're going. If you want anything specific, then just ask.