Nothing rails related, but general feedback.
Main Page:
Sign in on main page. This is the common use case compared to signing up.
It seems like the icons at the bottom should be links, even if it's just to tweaked signup pages.
Sign up page:
Your CSS seems messed up. There should not be a scroll bar as long as the sign in link is shown.
Logged in UI:
Margins seem off.
Needs filters, price range, location (look at sublet.com).
Should show your own listings.
Map view? (look at hotpads.com)
For a lot of rentals expect no pictures.
Icons need tooltips.
Absolutely needs lessor and lessee ratings and need to solicit these aggressively, there are too many housing scams out there these days.
Appointment UI:
Don't like it. Use a standard calendar control IMO. (e.g http://jqueryui.com/datepicker)
Appointment Workflow:
When I click an "attend an open house" button I expect to see when the lessor has set up an open house, perhaps with an "RSVP" and "request an appointment button". If the seller hasn't set up an open house the main UI should say something like "request an appointment". I assume that setting up an appointment fires off an email, but if it doesn't it should.
Concept:
Interesting, but crowded. Craigslist, hotpads, and sublet.com are all substantially sized competitors. You need to look at what they are doing and figure out how you are going to do it better, just focusing on a niche market isn't enough but does allow you to better in ways that the larger market (and your larger competitors) don't care about.
Business model:
I don't know about this one. Renters trash places, scammers double-rent the same apartments, and subletters get their crap thrown in the street when their "landlord" stops paying their rent and just pockets what you gave them... these are not situations you want to be in the middle of. Also the expectation is that you are going to pay a small listing fee upfront instead of paying monthly... you are fighting inertia on that one. OTOH, I can see why no upfront cash would be attractive to college kids, though a lot of them will just do a deal between themselves and not pay you.
Also keep in mind that housing is regulated to varying degrees through the country.