I bought a buck 501 over a year ago and I would basically use it for opening packages, mail, cutting up boxes, and string. I noticed a few weeks back that when I held the knife perpendicular to the floor with the blade facing down, the blade would hang a little. I sent it to buck and they said the spring is being fixed and will be mailed back to me.
I think your question is more about blade play and less about the 501 compared to other Bucks, no?
From what I've seen, you aren't the first to post to this forum with problems with blade play with their Buck. The search function may provide you with more insights. Buck moved from California to Idaho in 2005 and some regular posters to this forum (who know more about Bucks than I do, which isn't saying much) have asserted that the QC took a hit with the move, which is somewhat to be expected.
The head of Buck's QC department watches this forum and we know that these issues are of a high concern to them. One thing that has been incredibly consistent is Buck's commitment to customer satisfaction. While it's a bummer to have a knife loosen up with light to moderate use, I'm confident that Buck will make things right. Even with the best QC, you have to allow for materials on any hard good to fail occasionally.
(Edited to add: If you've been opening the knife by pinching the spine of the blade and flicking the frame open with a snap of your wrist, this may be culprit. Lockbacks aren't designed for this sort of harsh opening and it can lead to premature wear and tear to the blade/lockbar interface.)
Overall, I think Buck's quality is very good. Their production level knives might not have the "fit and finish" of top shelf customs or quasi-custom knives, but they have a great reputation for a good reason.
This said... I wouldn't expect their plastic framed lockers to be as robust as those with metal bolsters and liners and there are still open issues relative to premature development of vertical blade play under hard use (deformed bushing or pins, perhaps?). The folks from Buck have promised they will get an update out on that at some point.