I'm going to get one to help with pacing with some longer events this summer.

What's worth looking at?

Is it worth getting one with cadence??

Most computers do average speed, some have a little +/- arrow to tell you if you're above or below it. Personally I think cadence is a complete waste of time.
How much are you looking to spend? £15 gets you the basic wired thing with 8 functions, £300 gets you a Garmin GPS with colour screen and mapping!

Cateye always get good reviews…
http://www.chainreactioncycles.com/Models.aspx?ModelID=28239
(wired version is about £15)

I guess I'm not after GPS… GPS is to come later, maybe a christmas present to myself. The idea is just to help me hit a 9/10 hr average pace, so a variation from the long term/moving average would be useful. OR indeed a target pace.

Any opinion on wirelessness?

I dont bother with cadence. If you're happy with your current cadence, then why push yourself out of it in order to try and achieve a magical cadence value which people spout as being the most optimal.

As for computers … For the last number of years, I've bought Cateye enduro 8's. They've consistently been reliable, battery lasted ages and ages, the fittings reliable and solid. It also has a relatively large digits + a secondary set of digits, it shows a +/- arrow at the same time to show whether you above below average. Spares are easy and its not too expensive in the first place.

I cant see past them, just been great value for me. ( i get the ones with the thicker, stronger cable )

Another vote for wired Cateye. Have had the Enduros with the thicker cable, but then have never broken a thin one either – you shouldn't snag it if you route it properly. Wireless is more trouble than it's worth IMHO – really not that difficult to route a wire via fork leg and front brake hose. I also don't have cadence – if you really care then it's easy to work out based on gearing and speed – I have an idea of minimum speeds in each gear on my road bike to maintain cadence. Don't even think about it off road.

Always been happy with my Cateye Micro wireless but just had one go a bit funny on me so have replaced it with a Strada one which seems neater but i've not used it in anger yet.