I like my Cateye Strada Wireless. You can get one off Amazon for about $40 shipped, works on stem/bars, and you activate it by pressing the whole unit down rather than a button on the front (more rain resistant, and you don't have to aim for a button). Slides right in/out of the holder so you can take it with you when you lock up.

http://www.amazon.com/Cateye-CC-RD30...3982039&sr=1-1