I quit bottling entire batches long ago. It was tedious, and I do most of my beer drinking at home, so there wasn't much need for portability, and kegging became my standard. For social events, I took to using growlers. However, I still had an occasional need to bottle part of a batch to share with friends and family. Enter the Blichmann Beer Gun.
I bought my Beer Gun about 7 or 8 years ago when it was a fairly new product. It was the simplest, off the shelf solution for bottling directly from a keg, albeit a bit expensive, as with most Blichmann products. Over the years, it's likely paid for itself in convenience, although there are some very cost effective alternatives.
The included directions are straightforward, and the Beer Gun's basic operation is pretty simple: hook up a low pressure CO2 line and a keg of beer, push the button to CO2 purge a sanitized bottle, then pull the trigger to fill. The key to success is extremely low pressure in the beer keg. I purge all pressure from a fully carbonated keg, then slowly dial the regulator up until it barely registers pressure. Any more than 1-2 psi is a recipe for a foamy mess. Bottling in chilled bottles also helps keep the foam down, however I've never used the Beer Gun without having at least an ounce or two of loss to foam per 12 pack.
I've read complaints about getting flat or under carbonated beer out of Beer Gun filled bottles, but that is not my experience. Extremely low line pressure ensures that the beer arrives at the bottle in an overcarbonated state. Headspace coupled with a proper pour serves to balance the carbonation at serving time.
The little rubber stopper at the Beer Gun's outlet is its biggest physical flaw. It tends to fall off during cleaning/sanitizing and is easily lost in a foamy sanitizing solution. If the stopper is accidentally disturbed during dispensing, there is no way to stop the flow of beer other than disconnecting the line from the keg. Fortunately, that's easily prevented with a little care during use.
While the Beer Gun is a handy bit of kit, it is overpriced. With a few spare parts and/or about $10, you can build a nearly equally effective "BMBF" - discussed at length in this HBT thread. If CO2 purging along with minimal foam/mess are imperative, you can build a simple counter pressure filler for about 1/3 the cost of the Beer Gun.
Overall, I'm fairly satisfied with Blichmann's product. It works as advertised and offers plenty of convenience for the kegging-minded homebrewer. However, I can't wholeheartedly endorse it due to readily available, easily made, far less expensive options.