Shopping around for an external hard drive, tips, suggestions?

I'm shopping around for this to save some video files on my computer to it, as my library has become quite full and accounting for a lot of the space on my 500gb hard drive. I don't see this collection building into the 200gb range, so,

with that being said, i'm looking for an external hard drive that is easy to use, and under $100. Also i only have USB 2.0 on my computer, i heard it works anyway (using 3.0 in a 2.0 slot) but i don't want to pay for the latest and greatest.

I've been looking around, and the one i been lookin at the most is -

http://www.ebay.com/itm/Seagate-GoFlex-3%E2%80%A6

Is this a good choice? I've read reviews which say its not very user friendly, which for me is not a huge deal, as i know quite a bit about computers. But that it is very dependable, and sleek.

One of the reviews i seen, is that this model constantly prompts you to back up again, which i don't want, i basically want a flash drive with more space. Windows files i don't care about backing up, (or perhaps i will if i had enough space) most of the time if a Windows file becomes infected, i can just clear my operating system and reinstall Windows 7. Then start over from scratch, this may be a bad thing though, but its always worked for me.

I've also never used an external hard drive before, and had some questions as to how they work.

Do they stay plugged in constantly?

Or can i unplug them when i'm done moving things to it?

Will it lag my system at all? (not while i'm transferring, but in general and idling)

Thanks in advance.