I bulk load whenever the cost savings seem worthwhile, which is usually for B&W films that I use frequently. That said, since most of the posts to date have been unequivocally positive, I'd like to play Devil's Advocate and point out the downsides....
IMHO, bulk loading has two big problems: It takes time (and time is money, to one extent or another) and it increases risk. The time/money issue is one that's highly subjective. The risk factor was mentioned by BetterSense, but it deserves emphasis. I've scratched more than one roll of film in bulk loading. (Mostly from a bulk loader with a poor design that runs film over hard plastic curves -- that's just asking for trouble. I now use a different bulk loader with a safer straight-path design.) There are also risks of scratches from old cartridges, and of more dust getting onto the film. Depending on how you do it, you might be unable to avoid fogging of the last frame (or two or three or four) of each roll. Even if you do it in total darkness, bulk-load cartridges are more likely to pop open when dropped or otherwise mishandled than are commercial cartridges. I've had one or two rolls rip free of their spools at the end of a roll, which can nix further photography until you get home if you don't happen to have a changing bag or other dark area available.
Now, stepping away from my Devil's Advocate role, I'll say that these problems are pretty minor from my hobbyist perspective. I've only lost a few frames to these problems, and none of them have been important shots. Some issues, like end-of-roll fogging, are known and can be planned around. If I were a professional photographer, I probably wouldn't rely on bulk-loaded film, at least not for paid work. As a hobbyist, it's another matter. Oh, one other advantage that I don't recall seeing mentioned in this thread so far: If you buy a lot of film at a time for use over months or years, bulk film is more compact, and therefore takes up less space in the refrigerator or freezer, if that's how you store it.