Just finished my first rental and it worked great!
A few comments from a renters perspective (I didn't go through the whole thread so these may have already come up):
1) nice to have a past transactions page. It took me a while this morning to track down who I rented from and how long I had left.
2) can you set up a failover pool? I would be hesitant to rent an expensive rig for a long period without one.
3) an ability for past renters to comment on the rig they rented (and the ability for the owner to respond).
On that last note, I rented from AspireMining and it went smoothly, with advertised hash rates, etc.
Really excellent for such a new site!
Cheers