Similar experience here. At home, my phone has a good signal (on EE), but the car seems to have a poor signal. Often takes over a minute to connect - in fact I've increased my phone's display timeout to 2 minutes just to allow for the iSmart app's slow response.
I get erroneous data too. Often the sunroof is shown as open (when it's most definitely closed). Sometimes the internal and/or external temperature data are missing (often both). Sometimes the battery percentage is shown but not the range - sometimes the other way around.
But it was much more responsive when parked at Ikea, Reading today. My phone had a poor signal, but it seemed the car had a good signal. It appears to work much better that way round. Overall, it's usable, but frustratingly slow and error prone.