This summer I invested in a Gamefly account and I believe it really save me money, especially since I managed to get a free month twice with two separate accounts. But I was finally able to try out a lot of games that I didn't want to buy, and I was able to Platinum them quickly so I would send them back immediately once I did that.
Right now, I can tell I'll need to invest a lot of time to the Splinter Cell Trilogy to platinum it, so I might end up buying it if it goes on sale soon. Plus I really like the SC games and would love to have them as part of my actual collection. It's what I did when I discovered how much I loved Mass Effect 2. But there's a few games like Arkham City, Uncharted, and Assassin's Creed that I will buy automatically because I know what to expect and they are part of my collection already. I can't imagine myself ever selling those games, especially since I still haven't platinumed any of them.