I use CycleMeter. The speedometer isn't accurate for sure (it says I've been over 70mph when I maybe hit 40 tops). But the mapping log is awesome. It can even send updates along the way to your Twitter or email and at the end of the ride send you an awesome google map link with every mile marked out and detailed.
My wife also likes the en route updates when I'm venturing off the beaten path for an afternoon.
In all I'd say that you can't trust the iPhone's GPS for reliable speedometer info. Use a Cycle Analyst or get a regular GPS for accuracy.