Puk it - the simplicity of the puk is what makes it so useful. Use speed or heading on the display combined with the header/lift graphic; one quick glance is all it takes and trends are very evident. The download is gravy for the debrief. The fun is everyone displaying max speed at the end of the day and dropping it on the tramp to see who pissed biggest. I love my Puk.
What he said. I use mine to also satisfy the compass requirement for distance racing. I also have a GPS on me, but in general messing with it while driving is bad. Can read the big numbers at a glance. I took the $$ route and bought two, and stuck one on either side of the boom. Can easily hit the buttons to switch modes, nothing up front to hang on stuff. I've added the velcro to the cabin sides of my F-27 and use the dual Pucks there too - can't see the instruments from the floats, can see the pucks... There are some sweet tacking brackets that folks come up with to mount aft of the mast.
I have an SC-1, and it's nice but it never helped out much on the 20. We had a bracket that mounted to one side of the front beam. During Tybee the salt spray continually changed the settings, and then a wave took it off the velcro. Got to it as it went bouncing along the tramp. Another time at the end of an Annapolis-Oxford a spin set got a line on it and flung it up in the air and overboard. Man overboard! So the velcro can be defeated, lash things if you don't want to lose them.
When I have some time I want to do a gps overlay on some gopro sailing footage, so you can actually know how fast you go when you do what. I looked at the iphone as well for a sailing instrument and actually think its ideal, it has a gps, camera, data connection and a lot of other sensors like a gyro. Maybe also thinking about putting this into some sort of dashboard with a video overlay. If you have an iphone 3gs or 4 this free app registers everything the sensors do: http://itunes.apple.com/us/app/xsensor/id345145166?mt=8
A speedometer like the speedpuck looks interesting but speed alone would not give enough information without having something to compare it with (like maybe your speed on this side of the course on the previous lap or something).
Actually, I'd love to the same - but why do you think, an iPhone could be solution? First, I assume you just get two files (.gpx and .mov), which you have to combine and align somehow. Secondly, can you run an app and the camera at the same time? Third, how are you going to mount the phone secure and waterproof - in a way that you can touch the screen? I'll upload soon a video from my GoPro where we capsize into a turtle ... I have the camera and I have a Garmin 301 which supplies all necessary data. What I dream of is a 4 segment split screen: footage, speed (or VMG), course, and wind direction and speed. Remains a dream, I guess.
I have a speed puck but for race analysis I really like the Kattack approach: 1. Uses cheap GPS (does not work with Speedpuck) 2. Everyone turns in their unit so you can see how you compare to other racers. You learn a lot more than just looking at your own tracks. 3. You can upload the races to the web where anybody can watch them.
Velocitek has their own priprietary software that does the same thing, but it supports only the velocitk units and I don't think there is anyway to upload to the web.
Kattack is good for a club or people that regularly race against each other.