People in the shooting community always seem to have incredibly mixed opinions on paintball. It is my opinion that paintball is incredibly underrated, both as a training aid and as a source of fun and bonding. The above video shows elements of both.
One of the big problems people have with paintball is the ammunition, typically handled by way of what amounts to a bucket bolted to the top of your gun. By and large, this is the most common means of feeding paint into a gun, but there are several others; low-cap hoppers hold from 10-50 rounds and barely stick up from the gun, magazine fed paintball guns have greatly increased in both reliability and availability, and there are even a few things that split the difference between them, such as the qloader. The qloader and qpods are my personal favorite, because it is reliable but functions almost identically to a high-capacity magazine.
Another problem people have is the range and accuracy, or lack thereof. This is something that has recently been remedied, at least in part. A company known as Tiberius Arms developed what amounts to a paintball rifled slug, giving doubled range and 5x the accuracy of standard paint. The costs are significant (currently $40 for 100 rounds), but they DO work exactly as advertised. The biggest downside is that most paintball ranges are heavily designed to favor CQB engagements.
The next problem people (myself included) have with paintball is the cost. It's not uncommon for paintball guns to cost as much or more than the real steel version they mimic (for the guns that aren't space guns. Tactical reproductions have drastically increased in popularity). While it's possible to spend vast sums of money on paintball equipment, it is similarly possible to spend relatively small amounts, down to ~$60 for the most basic gear possible. It helps to consider this not as wasted money that could have gone to an equivalent REAL gun, but as training fee. Paintball allows for live fire against live opponents who are doing their damnedest to take you out in turn. It's one thing to practice using cover, it's another thing entirely to learn under fire how well you can shelter behind something 6" wide if you really need to.
Another thing about paintballing is the camaraderie. I've found myself pushing myself hard for complete strangers on my team, risking exhaustion and welts to help claim victory. By the descriptions I've read of the various Gunblogger Rendezvous, I'd have to guess that the atmosphere around many of the big scenario games is quite similar, with an added helping of Brothers in Arms. A vaguely local scenario I've gone to many times routinely has 1500-2000 players total, organized by generals into squads, fighting for objectives (capture the X, deliver a payload to Y, etc).
TL;DR version: If you haven't tried paintballing with friends, do so. It's worth the time and the welts.