Most will find it quite crappy because they don't have positioning down-pat. With ~20ish bolts spinning out of it at VERY close range most huge hp monsters become cakewalks.
The trick to trap positioning: right-click and hold your CBS on the ground (NOT ON THE MONSTER) and then quickly drag the pointer to a 10/2/4/8-o-clock position right up to the monster, and that's near-perfect positioning. Try it a coupla times and you'll get the hang easily.
I admit I haven't messed around much with placing these exactly. But even under the assumption that every bolt hits, it doesn't outdamage LS by all that much. The most common build is the 80 point LS-synergy trapper, with plus 10 trap skills you get:
CBS: 5 traps times 11 bolts, 4-532 each = 44-5852
LS: 5 traps times 1 shot, 8-5362
Even with all bolts connecting, CBS is not all that different from LS (10% better). IIRC the two traps cooldown times are the same, so that's a noneffect. I would imagine that the CBS traps would have more consistent total damage, however. (central limit theorem - sum distribution of many independent events approaches gaussian regardless of their independent distributions? rachbot off)
If you nail a boss with less than 11 bolts, you're certainly not coming out on top at all. And in general PVM play CBS is definitely worse because its bolts are absorbed while LS pierces. To make CBS comparatively better, you !have! to max fireblast, because FB synergizes CBS and doesn't synergize LS. Here's the numbers of the 100 point trapper (FB/SW/CBS/LS/DS) with +10 skills.
CBS: 4-708 times 11 bolts = 44-7788
LS: 8-5362
Once any sentry is maxed, I would almost always max one of the "chuckables" instead of maxing another sentry. It's additional active damage that complements your passive traps nicely (without chuckables, I think trappers get a little boring sometimes). Same idea as maxing timered and untimered skills and casting them independently instead of maxing two timered skills. Death sentry is the exception to the rule because it's so unique and tri-element.
it really is a ridiculously strong bosskiller even at low levels. .
Even at low levels I think FB is better though. At lvl 15, you could have 5 CBS traps doing 7 bolts of 1-21 damage (4 CBS, 6 SW, 6 FB), that cost 5*13=65 mana to drop. They'll shoot five waves and expire, leading to 35-735 damage, total, if every bolt connects with something.
With 16 points in fireblast, it does 45-65 and costs 4.8 mana. You could chuck 13 of em for 65 mana, and do 585-845. PLUS, it's an area of effect spell, so multiply that by however many monsters you can cram into a radius of 3.3 yards! I think FB gets no love because of its "time to flight" but in pure damage numbers it rocks. It demolishes A2 and A3 untwinked - herd up big groups of skeletons/flayers and use splash damage efficiently. Conveniently, it also trashes Andariel (naturally weak against fire).
That's a damage per mana cost argument, but you also can make damage / time arguments because fireblast can be chucked very fast using burst of speed (it's the same speed as normal attack) and CBS is limited by its slower cooldown. You probably won't have enough mana to spam either one willy-nilly though.
Man now I wanna make another untwinked trapper. :cloud9:
I like numbers.
