I've owned several. I had a Pact that worked well until a friend (?) drilled it with a 44 mag. I still have an old Chrony that works perfect and a dual sky screen Oehler. Oehler is the top of the line, Chrony is the bottom. They both read the same when I test them together.
The Chrony is very easy and fast to set up. It can be a little fussy about light conditions and has been known to spook when birds or clouds move overhead. I use this chronograph the most because it is a fraction the cost of my Oehler (they no longer make chronographs) should a bullet stray. I often will use both at the same time. I set the Oehler up at 12 feet and the Chrony at 100 yards. I can plug the results into Ballistic Explorer to compute actual ballistic coefficient.
There are several brands available and all are quite good. If you want the extra bells and whistles like a remote readout, built in memory, printer, or calculator, it will cost you more. A remote read out is well worth the money however, I seldom use the math functions or printer.
Basically what you want to know is: Does your velocity match what the book says? When you shoot a 10 shot string, what is the max velocity spread from highest to lowest? When your max spread is tight, you know your reloading techniques are working. You can compute standard deviation but that really is a fairly worthless stastic that is a prediction of what the next round fired should be. The ballistic coefficient test is great for charting long range rifle loads.
I firmly believe a chronograph is one of the most valuable tools you can have, especially if you reload. When I work up a load for a new cartridge, I always buy a box of factory ammo with the same bullet weight. My goal is to match the factory velocity and keep max spreads less than factory ammo. One thing you will learn; most factory ammo is over rated in their literature.