Trust me, while the Rubie's is an ok costume to start with it'll bug you after a while as it's not at all accurate and made of cheap plastic. It took me about a year to complete my Vader, it was more expensive than the Rubie's but I'm 98% ok with it at the moment and people are really impressed as it's really looking like Vader, not just a costume.
So, if you need a party costume, get the Rubie's as it's easy to get and fun to wear I guess, but if you're thinking gettin something special join the propden and see what great props are out there. It'll take time to complete a Vader and you'll probably never be finished but it's A LOT of fun!
Oh, and think about what version you want to get. All 4 Vaders are different. The Rubie's is a weird mix of OT (Mask) ant PT (rest).