D2D charges $5 for five hours of playtime, then if you want to keep the game, your $5 will be knocked off its price. The selection is very limited at the moment, with only Divinity II - Ego Draconis, F.E.A.R., Race Driver: Grid and Silent Hill: Homecoming available now. D2D notes that "our rental selection will grow in time."
While the PC game digital distribution market is fiercely contested, there are only a few players in digital rentals. Cloud gaming service OnLive offers 3-day and 5-day rentals on a large selection of titles, generally priced at $5.99 and $8.99 respectively for modern games. Digital distributor Green Man Gaming has dipped its toes in with $1 weekend rentals, though it charged full price up front and returned the rest as store credit.
Steam, the big daddy of digital distribution, does not currently offer rentals. It does, however, at times offer something better--'free weekend' events, where every Steam user can play the full version of a certain game for free over a weekend. So far, these have focused more on multiplayer games than those based around single-player.
While Direct2Drive's rental scheme is none too exciting at present, it may become a little more attractive in the future if the terrible trend of publishers not releasing demos of PC games continues--and if D2D can get a decent lineup together at a good price.