Just built up an alu hardtail (an old mmmbop frame with some revs at 120) and took it for a spin round some local woods. First ride out felt amazing after riding full suss bikes for ages. Felt really light, lively and responsive, just encouraging me to get out the saddle and work the bike in a way a full suss doesn't. Brilliant fun. Don't think my FS will be seeing much of the local trails for a while.
Do have a steel hardtail itch that does need to be scratched, but on short blast type rides a stiff light alu frame is spot on.
EDIT: Recommendations, if you fancy an alu one and can find an mmmbop frame, they are great. Rated for 160 forks too so should be pretty strong. In steel I was looking at a Soul, but you might want to think about a BFe or a Pig. There's loads to choose from.