The Vetta RT77 was my first bike computer. It's a cheap, very basic model that has worked okay, but there is major room for improvement. Personally, I would spend a little extra to get a computer with a bigger screen and more functionality. This computer gives you the basics: speed, ride time, average speed, top speed, total distance ridden on trip, total distance ridden overall. Speed data can be displayed either in miles per hour or kilometers per hour.
My biggest complaints about it are as follows:
1) The screen is very tiny and only displays current speed + one other item of information (ride time; average speed, top speed, total distance ridden, etc.) This means that if you want to see a data parameter that isn't currently displayed, you have to press a button to cycle to another parameter, then press until you get back to the one you had originally. A bigger screen such as is featured on Planet Bike models that shows most of the parameters you will use on your ride without the need to switch through them, is a big advantage compared to this model.
2) My biggest gripe -- to reset the parameters (clear the ride data from the screen and start at zero), one has to press the two buttons on the computer, located on the left and right, simultaneously. The computer is extremely unforgiving when it comes to this -- you have to press the buttons (which are cheaply made) perfectly in sync, or else the computer will only recognize one button input and will simply cycle through the data screens. As the computer gets used more and more, the buttons become less sensitive and more difficult to press together simultaneously for reset. And in cold weather, forget about doing it with gloves on -- it's totally impossible. I find this incredibly annoying and I would avoid this model simply for this trait alone.
Bottom line - I would not recommend this bike computer unless you literally cannot afford an extra 10 or 20 dollars to get a Planet Bike model.