I stayed at the Millennium for the 2003 race. It was pretty good, not as extravagant as they lead on in their advertising, but worth staying at. Fanueil Hall is just across the street, so plenty of places to eat at there and Bill Rodgers' running store is there as well. Two different T-station for the green line are about a half-mile away, one up behind the Govt. Center and the other at the Old State House ( Boston Massacre site). The green line is the easiest way to get to the race expo, to the commons the morning of the race to get on the buses to Hopkinton and back from the finish at the end. It's a quick ride to/from the Millennium to Logan International from there, but you can just as easily take the train as well from the Airport to the Govt. Center station.
I'm also running Boston again this year. I had a bunch of Holiday Inn points built up, so I'm staying at the Holiday Inn express in Cambridge, (just off of the Lechmere T-stop). The base price for a room there was about the best I could find anyway. There's another H.I. down closer to the JFK Library, another over by the Airport. WIth the train system, staying down-town isn't as big of a deal for me, as you can get to all of the places that you need - Hotels in town are just extremely pricy.
As for the Copley, I can't tell you squat. My best advice otherwise is to try
www.tripadvisor.com
if you have a question about a place, get a map of the Boston area and compare this to hotels that you may find when you web-search for them