-Amplifying a low B takes a lot, and it seemed too prone to getting muddy/lost/just not as nice sounding as I would have liked. EQ helps, but not enough.
-You now have another string to keep muted/dead/etc. It ended up becoming a new rest for me, and occasionally meant one string was ringing against my will.
-The actual B string is thick, and firm, and doesn't fret quite as nicely as it's neighbors. The B also wobbles like a spaghetti noodle compared to the others, string tension issues becomes all the more apparent when you get low tension strings/downtune/etc.
-My 4's are lighter, slimmer, and cover enough of the frequency range to get the job done (I play mostly rock, not much heavy stuff).
Some people love the 5'ers, but I found that I'm a sucker for a 4. It's still a great thing to try out, but maybe try to borrow one for a while before you commit to owning one. In the store it seemed novel and interesting, but after a couple months i was ready to get back to my 4.