I went to Islay for a week in August with my missus and 3 year old.
One word – Fantastic
We stayed here
http://www.kintrafarm.co.uk/
No mountain biking, (but I stopped at Dalbeattie and mabie on way up), but for solitude, wildlife, beaches and whiskey, its amazing.
Ive been to all the main Scottish Islands, and I would rate Islay as my fav. It just has an atmosphere that I cant put my finger on. Maybe its the peat bogs?
I had a couple of road rides on the Island with my mountain bike, but there really is no mountain biking, (if you find any, let me know 🙂 )
If you wanna know anything, like attractions etc, just let me know.