Well I've got some money to spend in my steam wallet and have been pondering one whether or not I should finally go out and buy the PC version of Oblivion. I've always owned the PS3 version and have basically gone through every quest in the game 2 or 3 times, so I would only be buying it for the mods. So I was wondering, are all these mods I've been seeing on the internet over the years really worth 20$? Or should I save my money for something else on steam?

Most definitely, provided you liked the game on PS3. You'll be able to turn it into a brand new experience.

Yes, especially if you obtain Oscuro's Oblivion Overhaul. Yay

It becomes a brand new game; much more challenging and rewarding.

I just got a PC and am debating between OOO and FCOM (which includes OOO plus three other overhaul mods).

yeah, go for it. If'n you don't like it just remove it. You got nothing to worry about.

Some of the best mods will change the gameplaying experience in a subtle yet completely thorough way, such as the Unofficial Patch or Natural Faces.