Tempted to go for a few days in July but wanted to know if it's worth hiring a car to investigate the island. Also Palermo sounds a bit crazy – is it bearable to spend a few days there? And if there's any mountain-biking then even better.

Says it all really:

But – it is amazing – my wife and i had our honeymoon there. Didn't do any biking, but looking at it , i'd imagine it's awesome.
Lovely very un-developed country, bit rough around the edges – but all the better for it.
Skip Palermo and drive west (defo hire a car – but be warned, driving there is mental, but funny), head for Castellamare del Golfo, or better still, Scopello. We went in early October, still hot, dead quiet and stunning. Food's ace too!!

Pics below of Scopello and the nearby Zingaro national park (they filmed part of Oceans twelve here), beautiful. If you like snorkelling/diving, the seas around here are abundant with fish and the waters crystal clear.

I've done a fair bit of travelling, and Sicily is my favourite place. Been around the island last summer, and have gone back a couple times since. As el says, skip Palermo and drive west. He's right about Zingaro, beautiful place, and the archeological sites on the island are incredible. Segesta is one that's close to Palermo/Trapani/Castelamare, and right around the bend there is an agriturismo that you can sleep and/or eat at, and which to me is the most relaxing place in the world: Pispisa. They don't speak English, but the food is great and you can just lounge about or walk around the hills overlooking Segesta. It's well positioned for the north west part of the island.

I could write many pages, but should do a bit of work instead. Just one more thing:

I'm a bit weird, but for me Poggioreale Vecchio was one of the most memorable sights — it's a town which was abandoned after an earthquake a few decades ago. It's worth ignoring the no trespassing signs, totally surreal.

is it an expensive place to be? compared with the Amalfi Coast etc?

i've never been there, but after these pictures… i will. for sure. In Italy I know just the north. I have that went to sicily, and they liked very much. good choise i think.