I use two of them, both 3TB. One as storage, the other as backup.
3TB is a ton of space. Unfortunately, this also means that if the drive fails, you stand to lose a ton of data. Hence running two. Always have a back up. The downside is obviously having to spend double. But if you value your files, it's worth it.
Just last week, someone on here had a 2TB GoFlex fail and lost 10 years of family pics. Hard drives aren't fail-proof. Browse the reviews at newegg and you'll find that regardless of manufacturer, internal, external, etc... drives can fail.
I actually purchased the 3tb one off amazon cuz it was on sale for 149.99.
Per one reviewer's suggestion, I ran a full checkdisk on it and found unrepairable sectors on it. I was bummed out cuz it took 31 hours to do the test...but I sent it back to amazon right away and they're already shipping me my new one. Hopefully it'll be ok.
I also recommend getting a second external hard drive as a back-up. You can kind of predict when your hard drive is gonna fail, but it's always nice to have that added sense of security knowing you have a back-up.