Awhile ago I bought an MFJ Powergain mag mount antenna to connect to my Yaesu VX-5RS 2M/440MHz/6M HT while in my truck. Unfortunately, it didn't work well for me in that role, vibrating itself to uselessness in short order. So, a couple of weeks ago I ordered from Gigaparts a Diamond MR77 as a replacement.
My first impression of the Diamond upon opening the package was favorable. It seemed better made than the MFJ. E.g., where the coax goes into the base it's reinforced with one of those flexiible rubber things like you see on some power cords. When I screwed the antenna on it seemed more solid than the MFJ.
One thing I did the night before putting the Diamond on my truck was to seal anywhere it looked like water might be able to get in. Based on my prior experience with mag mount antennas like the MFJ and a couple for my CB, water can get in through joints and then you get rust, causing performance to deteriorate. The sealant should also help keep the antenna itself screwed down tight to the mount. I used liquid electrical tape as the sealant.
Yesterday I finally got to try the MR77 and I am very pleased. I was able to hit the MARC repeater in Paoli, PA from near Willow Grove, a distance of about 20 miles, using 5W. I had a QSO with another MARC member on my drive home and while it wasn't full quieting, the audio was perfectly intelligible.
Aside from vehicular use, mag mount antennas can be used at home or in the field if you put them on top of a metal object to act as a counterpoise. For example, you can place it on top of a metal cabinet, an ammo can, a steel garbage can lid, or a cookie sheet.