I've been using a PerformanceBox for around 2 years now, because I wanted realtime info whilst im on the track rather than waiting until the following day to see natsuck times.
I like the Qstar that Eric uses, the software looks brilliant and very easy to use and get information out of but the problem is thats 15mins after you did the laps and what did you do different on your fastest lap compared to the other dozen laps for that session? None of us are F1 drivers, you won't remember if you approached a corner with 5km more speed or braked 10m later or used a different entry line it simply won't happen. But using the split times from the PerformanceBox, or even better the predictive lap timing all live infront of your eyes as you are out on the track really makes a difference. I glance at split 1 and 2 and then the total laptime every single lap, I can then brake later into 1 corner and immediately see if my split is faster or slower and adjust accordingly.
The software is very informative but I find I have to watch the tutorial videos each time I use the software to remember how use the data properly. I also find that matching the data up with video to get a data overlay like you see on YouTube video's quite difficult. But this is likely just me not knowing how to use the software properly, or more so, only using it once every 6 weeks.
To combat the data overlay issue I recently bought a Contour+ (HD+GPS for about $600 with accessories) so that I get instant data overlay with Dashware software. I used it last weekend but unfortunately being my first use of the actual camera I had the lens set to 90deg instead of 0deg meaning my hour of video was a complete waste of time. Rotating the video in VLC doesn't fix the aspect ratio and no matter what I do it just can't be fixed so i'll have to test it at the next track day.
I video every session for 2 reasons. So that my family can watch what I do and also because watching your sessions back helps you realize what you are doing right and wrong by having the laptime and speed data on screen next to you.
Despite having both these pieces of equipment im still not a fast driver but I do have a massive amount of fun and that is the reason I attend 8+ trackdays a year.