My regret was buying a DAT machine that I never really used a lot anyway. I kept mixing down to my 1/2 track, but thought the DAT was the new kid on the block.
All other hardware, I don't ever regret using, because I pretty much knew what I was getting before buying....at any price break.
I also think the idea of REALLY expensive stuff is that you need the ENTIRE chain to be equal to itself....meaning a $3000 mic into a M-Audio preamp isn't going to reveal the details, when plugged into something nicer. Plus you probably won't hear it on your $200 Alesis monitors either.
There is also something to be said about the abilities to use, know and hear the small differences when you get into upper end stuff.
My problem is I have quite a bit of rack gear...old reverbs, digital delays, mid level compressors, EQ's that I really don't use anymore as I prefer doing it all in the computer. I could sell them, but for the pennies they're worth, I'll just keep them in case they find a use.... ... and realize I'm not talking about LA2A compressors, Eventide harmonizers...