They aren't as great as they seem, in cosmic terms of course. It's entirely reasonable with realistic improvements in current technology to cross the galaxy in a million years or so. Basically you'd need a 100 fold increase in rocket exhaust velocities. I think that'd be easily achieved with nuclear based propulsion.
How do you stay alive, where are you headed, and would your spacecraft last that long? Also the faster you go, your radiation exposure will increase.