I happen to agree with those who say the heart rate and cadence are very important if you are most interested in your level of fitness but is you just want to know how fast you are going which is all some folks want to know then just get a cycling computer you like - I prefer wireless for ease of installation and there are no wires to mess up the look of ones bike. The Garmin 705 GPS is a great cycling computer that will collect all the data most of us need, heart rate, cadence, speed, avg speed and a lot more it will also make a record of the route you rode and show you where you are on a map but is you do not want all that then I would not spend the money. If you are a data nut (I am) then take a look at the VDO Z3 or Z1, you can see the
Z3 HERE I use one of these because I have not found a cycling computer that collects more data and let you download it all into your computer.
I am a real nut about the data - I use a Garmin 605 (I would have gotten the 705 if I had know better) a VDO Z3 - why - I like to record my routes and I prep programs ride routes into it to follow before most rides if I have the data - I can make a ride route to follow from the data the GPS records on a ride. The Z3 shows me my heart rate, speed, altitude, % grade, temperature and one other line of data that I can choose like distance, odometer, watts, avg speed, time of day , time of the ride and about 20 other pieces of data like cadence.