It was either VonDondu, or myself, or both. Jan's a personal favorite of mine. From an RP perspective, he's a delight, and this continues on into ToB. From a party perspective, he's not just a very competent thief, but also a good second string mage. He's lousy at melee, just as you'd expect, but otherwise an excellent resource.
Incidentally, there's a large, fun piece of BG2 fanfiction entitled
Cards Reshuffled, that uses Jan quite a bit. I think the device of having nearly everybody cringe whenever he tells a story is waaay overdone, but the stories themselves, all by the author, are highly amusing.
Note that the TweakPak offers you the option to let Jan create another type of ammunition, as well as one or two items that are very carefully measured so as not to be too powerful. (In fact, one of them, as I recall, does nothing whatever, except offer you a slight chance to be singed by lightning.)