To add one more thread, I thought it might be interesting to list the games I don't regret buying, especially the ones that are not so obvious, or perhaps the ones that were bought, played (or not), and no longer played or even gotten rid of.
I certainly don't regret buying OD&D and AD&D. Got many years of good play.
I don't regret buying Fantasy Hero even though I just got rid of it last week. I did have an enjoyable game with it, but realized why it wasn't a good match for my interests. I learned something about what I want from a game from Fantasy Hero.
I don't regret buying Dogs in the Vinyard. I originally purchased the PDF as an example of a game that really laid out what the game was about (not the setting bit, but what the actual play experience was about) and how to play the game. I ended up trying it out, and enjoyed it. I may not play it for a while, but I will play it again.
I don't regret RuneQuest. I've had some lousy gaming with RQ, but I've also had some really good gaming. Plus, the chat I had with Greg Stafford when I had him sign my 1st ed. copy at Origins a few years ago was probably worth the $8.40 I spent on the book, and the incremental storage and transport costs that are part of the TCO.
The games I regret are the ones that never got played, or worse, never even got read. These are examples of games that I really did get my money's worth out of.