I use a Hager extension. Though not uncomfortable with using a bridge (been playing long before I ever saw an extension, lol) the extension comes in handy at certain times, especially if there is a problem placing a bridge because of clusters etc. I've also used the extension on the cue in conjunction with the bridge, this sometimes allows me to stroke the ball in the normal fashion with my arm in the standard position rather than the "butt plate in the face" mode normally associated with using a bridge.
I have a Hager and a Tony Ryan.. I keep the Hager in my case and the Ryan stays at home, next to the table. I often use these with the bridge and cue like a normal shot.. Helps me out a lot.
I use one when I'm not too lazy to get it out of my case. I bought it from Billy Palmer about 4-5 years ago when he came around Denver. I have no idea who makes it but I like it, it is very sturdy black plastic that tightens around the butt of the cue once slipped on. It also fits in the large pouches from many case manufactures: Instroke, Whitten, GTF, JB Cases, Sterling, etc., so it's always accessible.
i almost even hate to say this cus its too good of info to give out for free (i know, i think so highly of myself)...... but 1 hour of practice with a bridge will improve your game more than 10 hours of running balls.... that is if you have already been playing a long time.
I'll use one (I have two different kinds: Longoni and Tony Ryan) on rare occasion. I prefer the mechanical bridge as I'm pretty proficient with it. If the cue ball is "just" out of reach to make a decent bridge and there are no balls blocking my bridge arm, then I'll opt for the extension.
most days i play with my mezz. it's a retired model and i got the mezz extension for it before they got harder to get. i use it over the bridge but i still end up using the bridge more than the extension.
imo mezz did a good job with the extension. i think t he strongest feature of it is that it's big enough to get the job done but still fits in my back pocket when i'm not using it. once it gets taken out of my case i put it in my back pocket and leave it there just in case