I have this SAME exact setup and it's great. However, the only reason I'd tell you to hold off is because iPhone 5 comes out in September and will no longer use the current 30pin port. Therefore you won't be able to use this device until an adapter is released, and I'd imagine the hardware would get somewhat bulkier.
Aside from the obvious reasons as to why this device is great, one thing people forget is that by using this attachment, you can keep your phone in airplane mode (since it relies on this external hardware for GPS lock). That makes a big difference in battery life. For example, I was at Summit this past weekend where nobody really could get a signal. Even though I didn't use the phone, the fact it was constantly looking for a signal drained my battery from 90% to 35% by 4pm. Next day, I left in airplane mode, used the emprum on all 4 runs and still had 70% battery left.
Another device I suggest you getting is the GoPoint BT hardware. It'll let HLT sync up with your OBD so you can get engine data. Version 16 of the software should be released by August and the GoPoint will be compatible with HLT by then.
My best run at Summit Main:
(battery shows 31% but this was much later in the evening after I left the track)
