I think the Beer Gun is probably the easiest bottle filler I ever used. I've tried maybe four different CP fillers over the years, but the BBG wins hands down in ease of use and performance.
I got it as a gift (from someone who wasn't just spending my/our money, so it really was a gift), so I didn't have to make the decision if it was worth it or not. I guess not, since I didn't go buy it myself. But I rarely bottle a keg's worth of beer. I don't know that I'd break it out for less than a half keg's worth of beer, though, because I'd spend any time savings cleaning the thing afterwards.
You can rationalize not buying it on cost, but I don't think you can on engineering, manufacturing, or performance. It works exactly as promised.