Have the Boardman Altimeter myself, came with the bike (hybrid second hand but unused).
Wouldnt recommend this model (the speedo that is), its less reliable and consistent, i.e. spurious readings every now and again, than the wireless cheapo I bought from Argos a few years ago. It has a habit of resetting itself sometimes when I switch it on; also gave me a max speed of 47.7 mph on my ride home from work the other day. I know this bike is quick, but, come on.....
My point is I think you dont have to neccessarily go for expensive to get a reasonable buy - at c£40 the Boardman is not what I would call cheap, but give me my old Argos special (about a tenner) anyday!
Incidentally, both are wireless which are, I believe, prone to RF intereference and therefore less reliable. Never had any probs with the cheapo, though.
Hope this helps.