We have several of them here at home, and enjoy shooting them a lot. You can get very proficient in a short period of time, and some are quite accurate. Do not hunt with them in Idaho, but drew crossbow tags in Texas a couple years ago on a WMA in South Texas. We have killed WT, javelina, and hogs with them. Several friends in Texas use them on their leases all the time and love them.
As mentioned, they can be pretty noisy (loud) and a different much sharper noise too. If the bow utilizes string stops, they can be REALLY loud. The recurve -type such as Excaliber lend themselves to string dampeners better and are much better. The older wide compound x-bows (like Parker and Barnett) have enough straight string that you can put string silencers on them also. The newest state of the art reflex bows (Tenpoint or Raven) are too narrow and all the cable wraps back thru the pulleys. They are betting that extra speed will overcome the noise factor.
As far as the noise and spooking the animals, I think you need to look at reaction times. There is a lot of info on human reaction times, and it seems we react in about 0.17 sec to audible stimulus and 0.25 seconds to visual stimulus. The has been studied for decades in response to driving and accident scenarios. There is very little actual scientific data done to support deer reaction times, but it is believed to be about one half of ours. Call it 0.10 and 0.15 seconds for round numbers. Add in excellent perifreral vision and the use of lighted nocks (in low light) and they have some more advantage.
Using known speed formulas and gravity, with really loud noise and a keyed-up deer could move/drop as much as a foot at 30 yards, before your 350-400 fps crossbow bolt would arrive! Hogs and Javi's are stupid and don't seem to react hardly at all. They are the perfect targets for x-bows!!!
I'll keep using my vertical bow for deer and elk, thanks.