Haven't used one, but I've been looking around at what Plantronics has to offer more recently. I'm thinking about getting something for my sister as a birthday present.
I think the M155 is the model also known as the "Marque." And believe it or not, this unit is already considered old. There's a newer model which came out just this May called the M55, and it might be considered the M155's successor.
I don't really get Plantronics' marketing. They seem to always be coming out with new models without relating anything to models that already exist. Two year old models like the M100 & M1100 fall off the map (neither got very good reviews) and show up in the bargain bin, then you see new models in a lower price range with nothing directly related to what came before. Anyone who reads this sub-forum over time knows about the Voyager Pro models, as well as the Discovery 975. These two lines have been the high water marks for Plantronics, and are still available (higher-end models in the $80-$100 range), but they still keep coming out with new stuff in the $25-$60 range every year, seeming to throw stuff at the wall just to see what sticks (sells).
I think the M155 would be an okay model (especially at the price available from some online retailers), but I think I'd like to check out the M55, mainly because it's newer.
Regarding online retailers, it's a funny thing: You can get new, in-box versions of three and four year old models at some of these places (with original warranty), and if you happen to like the model offered, you're all set. And the prices are pretty good.