I bought aSmart HUD about 18 months ago. I used it mostly in the S2000. Since I bought the SRT6 I've been driving almost exclusively in daylight. The app works well enough. The S2000 has a digital speedometer so it was easy to check against the HUD app. Except for a slight lag, it was pretty accurate. I like the altitude info too just for kicks. Plus you can set warnings for a certain speed or speed limits as a visual reminder to slow down. And you can customize the look with three or four layouts and color schemes. Stay away from the Nav version. It is slow as hell and buggy. And the iPhone screen is really too small for nav info. My Garmin portable kicks the phone's butt in every way for navigation.

 

Remember the old saying "Jack of all trades, Master of none"? The Garmin does one thing, Navigation (yes it does music and bluetooth etc) and does it very well. I'm a Blackberry user and our recent trip to the Dragon was made much easier with our Garmin. It took a few minutes aquiring satellites when in the mountains, but worked all the time.