I use a Sigma BC 1609 + Cadence . I've got a second bike sensor kit ready to mount on my second bike.
Why do I use a Sigma ? Mainly because almost all the LBS where I live sell Sigma and use them themselves on their own bikes and for no other reason.

There are lots and lots of good ones out there Cat Eye, VDO etc. They're all easy to program and they're not very expensive. Ask at your local LBS what they use.

I've got a Garmin 305 Forerunner but I use it mainly for HR and intervals for x-country skiing and hiking. I may swap out the Sigma and put on a Garmin cadence sensor at some point but I prefer the Sigma because it has BIG numbers and I don't have to worry about the battery life on the computer itself.

The 305 always needs a recharge after 8-10 hours. The newer ones are supposed to be better. The Sigma needs battery replacement once a year.

Just make sure that the mount is solid so that whatever unit you have doesn't fly off and make sure that you use zip ties instead of those crummy O rings they supply you with. Attaching your smartphone is a good idea too as long as you have a weatherproof cradle for it. Some phones are allergic to water and don't do very well when they pop out of their mounts when you hit a big bump and they attempt to fend off an SUV.

One more thing about usability is to make sure that you can get at the data fields you want quickly and easily while you're on the bike with gloves on. I don't know of any bike computer where you can't do that easily but you never know.

You may only want the basic functions for now but I think that getting one that measures cadence is really worth the extra $$. It's one more sensor and it costs a little bit more but it's one bit of data that is really useful.

I got the second sensor kit instead of another whole package simply because I have enough gizmos as it is and I don't need anymore to keep of.