Agree with both CheezburgerBrown and MoonlessNights.
I was at a good paying job (for the area), but the work was ossifying into maintenance mode for internal-only apps; in 3-5 years, they wouldn't need anyone who could do put together new or better systems. Being a government contract project (federal level), I figured that 3-5 was about how long before I was on the chopping block or eyeing water towers as a sniper.
(Add to this the fact that the old IT team from 30 years ago was still around working on another part of the facility--it was like getting drug along by the Ghost of IT Departments Future, and I didn't want to become a caricature of myself or them...)
So I started looking around, willing to take a small short-term cut for long term growth and happiness. What I ended up with was more pay, working from home, and an entire industry that was ripe for upgrade and improvement.
You've got a job, so take your time. If the one you're thinking about will make you happy, and has the upward mobility you want, then you'll just have to make the call. If it doesn't pan out (like about 1/2 dozen of my potential jobs did), just keep looking.