I was thinking about buying a seven string guitar but i've never played one is this a bad idea
A bad idea?,,,,,no, I wouldn't think so. It might take a little time to get used to, but it's that way with any new guitar.
to quote dimebag darrel "I dont see why some guitarists need seven strings when they cant play on six"

Which is only his opinion, sevenstrings are pretty fun.
do you find yourself detuning or tuning to thins such as CADGBE? if so, you could use the range.

now a seven string baritone, that'd be awesome.
sevens are fun, espically with a half stack. those palm mutes = th3 $ex

but yea, it takes a while to get used to, but once its down, you probably not want to switch back (i got a 5 string bass, and i dont want 4's anymore)
i got a 6 string bass and dont want 4 or 5

looking into a 7 string guitar soon tho :P