If by "handheld" you mean holding it in your hands as opposed to on a tripod, then the answer is yes. It's not a light camera by any measure, but it's not as heavy as, say, a Mamiya RB67.
It's basically a Polaroid-branded version of the Mamiya Universal Press, with similar (but totally incompatible) lenses and accessories. The "native" film for the 600se is Polaroid pack film, which is no longer available, except as expired packs at high prices. It's possible to fit 120 roll film backs, either the Mamiya 6x6/6x7/6x9 "S" backs or RB67 6x7/6x8 backs. It's also possible to shoot 4x5, although the coverage isn't quite full frame (it's fairly close though).
All in all, it's a very flexible camera... but the adapters required to fit 120 or 4x5 backs are expensive and uncommon.