SEGA Saturn/DreamCast - got these later in the PS2 generation (2002/3) for a few of the critically acclaimed games that they offered. In hindsight these were bad purchases due to there not being enough games on each machine that I wanted/could find in order to purchase; not to mention that most of the games I bought the console for have since been re-released on more modern consoles (SF3:3rd Strike on DreamCast, for example).
N64 - bought primarily for the so-called 'greatest wrestling games ever'. What I found were outdated, slow, games, where you puffed your chest out to block which looked rediculous. Wasn't on par with SD2, let along anything that followed. So glad I was a PSOne kid back when N64s were current gen.
GameCube - bought for the Resident Evil exclusives. Freaking HATE the GC controller. To the piont where I've still not completed the RE series I bought it for. A complete waste of money since RE0 turned out to be not that great, and REmakes of RE2/3 never happened.
PS3 - I think Sony killed themselves with the high launch price of PS3, people said 'forget that' and bought the cheaper 360. Then when PS3 did release not only did people already have friends on XBL or they had already paid for games/addons DLC, but upon launch hte PS3 didn't even have cross game chat (if I remember correctly), so people weren't likely to leave the XBL community if they were already there for what amounted to a lesser experience. Fast forward in time a little and Sony remove PS2 backwards compatability, first in part by removing the PS2 'emotion engine' hardware and relying on software emulation, and later removing it fully; and thus PS3 lost another great boon that it had. I certainly regret getting my PS3, I was hyped by PS3 exclusive content such as Joker in Batman:AA, which turned-out to be a total let-down. I keep thinking that PS3 is lucky it is currently the only console with BluRay functionality inside it, as I'm sure many people bought a PS3 to function as their first cheap BluRay player, rather than buying a dedicated machine. Since Xbox is trying to become a media center now, I assume Xbox720 will use BluRay also, and there goes Sony's last 'one up' over the Xbox brand, outside of free online (which seems to be more as case of "free = shit" with the amount of time PSN is down for maintenance/hacked; and Sony may well decide to make that into a pay service at the start of the next console generation anyway).
Wii: I knew Wii wouldn't be my thing, it was bought mainly as an exercise machine for the fiance. But I thought I might get some fun out of, say, Resident Evil Umbrella/Darkside Chronicles (which eventually came to 360). Turns out I am completely inept at motion controls, and the fiance barely played on the machine. She now, in my opinion, wastes £30 a month on a gym membership which she goes to with a co-worker friend.