I started out with a unit that required me to shoot a bullet through wire screens, but for the last decade or more I've been using a Shooting Chrony with the now common light sensor technology. This company makes three models for shooters: Alpha, Beta and Gamma. Each model has a different set of features, but all share the same folding box design that's compact, easy to set up and a bit of a company trademark.
The Alpha is the most basic design and will measure velocities as well as compute averages, extreme spread and standard deviation. The Beta will do all of this, but will store data as well, allowing it to be downloaded later into a computer. The Gamma adds a timer capability, in that it records elapsed time between shots, up to a rate of 1,800 rounds/minute. A corded remote is available for all models, as is a printer capability for the latter two.
I own two, a Beta and a Gamma. The Beta is a recent acquisition and it now allows me to calculate ballistic coefficient. With the two chronographs set a known distance apart (usually 100 yards), I can measure velocity loss over distance. It's a revealing exercise to see how much the bullet maker's estimates vary from real world conditions.
Back in the dark ages (before the year 2000), when chronographs were delicate and expensive, this wouldn't have been realistic. But with a price point that ranges from $100-$200, the Chrony line is priced low enough that a shooter can own more than one. Even at that price, these units have proven reliable, as I've never had a moment's trouble. Change the single 9V battery every few years and they just keep on running.
Like any chronograph using light sensors to start and stop an internal clock, the quality of light above the sensor is important. I always position the diffuser panels above the sensors and make sure they have a clear view of the sky above. The corded remote, which Chrony has available, is a great option and it's the one accessory I think is a necessity, especially if you shoot on ranges with multiple shooters on the line at once.Having the remote on the bench with you means being able to review, delete and organize your readings without having to go forward of the firing line. The printer is nice and I have one, but rarely use it. I'd suggest that for most shooters the basic Alpha unit with a remote is the way to go.
If you don't own a chronograph yet, the Shooting Chrony line is a great place to put your dollars. A decade of use has taught me their product is dependable and a great value.