The 551 is the knife I carry most often, although it is my designated "tactical" folder, so it sees no actual use. I chose this knife for this purpose because the handle fits my hand perfectly - it has a very full and well-rounded grip that will work in almost any grip style. Also, I have a lot of confidence in the Axis lock design. Because the grip doesn't have any pronounced cutouts or other features, it should probably work for most people, regardless of their hand size. [I should note that I removed the pocket clip.]
rev_jch thinks the knife is a little bulky, but it definitely is not heavy for a knife this size. I wear a suit or coat-and-tie most days and this knife doesn't bother me at all in my front pants pocket. I've heard only good things about BM's 440C from forumites who seem to know about such things, so I have no doubt that this knife would be a good using knife, and, as NGK points out, this knife is a good value, considering the practical design and Axis lock.
miguel