I have a regular 250gb 3.5" external, I've had it for about 2 and a half years.
the only time i've ever powered it down is when i'm away, or when i'm moving it.
I leave it on day and night and it's been fine.
I also have a lcie 500gb, i've only had it about 6 months, but I also leave it on, it's a lot quieteras it actually spins down after about 10mins on none use, which is great.
my only peeve is that if I have itunes open, and don't se it for 10mins or so, then play a song, there's a few seconds delay while the lacie spins up again, but after that it's great
I recommend leaving it on, as it spins down when it needs to, but more impotantly than that...back it up, back it all up.