I have the Wahoo fitness case. It's ok. The mount is a little finicky and can rattle if you are not careful to keep it clear of the ends of the stem. Other than that, it works ok. This is the case with the ANT+ support in it.
From the iPhone 4S on, they support the new Bluetooth standard (4.0?) that works with low power sensors. Wahoo Fitness just released a bluetooth cadence/speed sensor. That would be the way to go.
I would like to see Wahoo incorporate a barometric sensor in their case like Garmin does in their bike GPS to help deal with the altitude issue on the iPhone. THe sensor can be use to smooth out the fairly erratic altitude on the phones for altitude (and hence grade etc...).
J.