First of all, 5.5" is hardly a "bad size". Sure, no one's ever going to call you "huge", but 5.5" is, at the very least, the low end of the "normal" spectrum. It's hardly what I would call "sad", and there's no reason you should feel so bad about having one.
Second, trust me-- a master in the bedroom could still send shockwaves through a girl with a 5.5 incher. Unless you're officially in the small range, as in the 4 inchers and below, you're not going to have any mechanical difficulties that can't be remedied with practice and savvy.
Lastly, even if you gained nothing more than an inch, that'd put you at 6.5", in which case you'd then be above average. So you've got nothing to lose really. And, if you really do manage to grow a good 2 inches or so, you'll be well above average and comfortably in the realm of "really big".
I'd say it's definitely worth it, but even if you don't try it, you've got no reason to be so down on yourself. Whether you're 5.5" or 7.5", there's a lot you can do with that thing.