i have been there twice!

It is my favourite nature spot in Japan. Don't know about the weather in july though...

i was there twice in may. saw turtles, monkeys and deer.

I stayed in Onaida. A couchsurfer has a camping/hostel there. wifi internet and a computer you can use. they are very knowledgeable. They have snorkels for rent! Onsen in town, bikes for rent at the shell station, supermarket to stock up. you can do a tough but amazing hike from behind the onsen up to 1400 meters. I encountered no one!
But if it rains i would advice against it.
You can buy a two day bus pass at the JR hotel in town.

Check it out: http://www.couchsurfing.org/profile.html?id=KS38MU

and: http://eu-guesthouse-in-yakushima.net
Rainer isn't there now, but his japanese girlfriend knows a lot too!

It was wonderful. I would count as one of the most beautiful places I've been. I was really lucky for the weather, not a drop the entire time. I didn't make it Onaido as suggested because I was naive and thought I could bike around the island to everywhere I wanted to go... wasn't the case. But not too far a biking from the Miyanoura Port (where the ferry lands) is the Kugusawa trail, it's pretty, midly difficult, and best of all secluded. It eventually joins up with a main trail (Siratani I think it's called) and offers an alternative route to the really old cedar tree besides the common follow the railroad tracks one that everyone takes and is really crowded. I took two days to hike it and stayed at a tenting area really close to Jomon Sugi so I could get up really early and experience it before the people started showing up. You can also take a bus to an onsen on the south side that is right on the beach. It's open air, and only costs 100 yen (well... I guess you have get to it so add in transportation whether it's bus, rented car, or godsforbid rented bike) and really picturesque.

Overall the whole island was amazing and really humbling.