I was under the impression that Mahab. is shut during the monsoons. Yet I went there in June 2004 and it was just awesome. The place is permanently covered with fog and it is a cool 10-15c. It rains very heavily but you do get a couple of hours in the day without rain when u can go out and enjoy the fog and cool winds. I have been to Mahab. in all seasons but I found it the best to go in the monsoons.
U can get great discounts (upto 70%) in the hotels. No need to book in advance. Just go there and bargain.
Try the corn pattice at the road side stalls. It is yumm. Also have the pizza at mapro Garden which is 7-8 kms away from Mahab. towards Panchgani.
THE ELECTRICITY GOES ON TUESDAYS. MORNING TILL EVENING. SO AVOID GOING THERE ON A TUESDAY COZ IF U HAVE TO STAY INDOORS IT CAN GET VERY BORING WITHOUT THE TV.