Hi shanghai,
Have a look
here for threads about hiring a car in Phuket. Some good info here.
The roads are not too bad, very very hilly around Patong in particular, and you will have to get used to the type of driving you have never experienced before
All I will say is, be prepared for the unexpected. The Thais are notoriously poor drivers, will use all the lanes turn without signalling, turn across or in front of you, so just be prepared.
Wherever you do hire, have a good look around the car for any obvious damage, scratches, dents etc, and point them out and even take a photo of them whilst at the hire firm. This should help you should they decide a previous dent was caused by you
Good luck and happy motoring
