It's easily done, pebs, but you have to take care to arrive on the island at the right time. We got there in the early evening and by then the buses had all finished for the day. If you get to Stornoway before, say, half four in the afternoon then you'll be able to get a bus straight to Callanish - there's one roughly every hour. The bus station is dead close to the ferry terminal and it'll drop you off in the visitor centre car-park at Callanish. If I were you I'd stay on until it gets into Callanish village so that you can do the last mile or so on foot. That's just me, though - I wish I'd approached the monument from the avenue end (thus avoiding the visitor centre), and I like to walk the last part of my journeys to the stones in any case. I've heard that it's MUCH cheaper to take the car if you get the ferry from somewhere other than Ullapool, but I can't remember where. I have a feeling that involved going via Skye.
It'll blow your mind! Be sure to see Cnoc Fillibhear Bheagg - that's the place to get a real feel for the Callanish landscape from. The main monument usually has a thousand camera lenses on it.