I absolutely love the accessport, and wouldn't switch to open source. Colin (wrinklechops) has open source, and after us flashing his car, and seeing what it entails... it really dissuaded me from ever contemplating opensource.
I really like not having a computer in my car, and I really like the gauges. I use the boost gauge religiously. I've been on a dyno 3 times and it's been within 0.1 accuracy of the tuner's boost gauge every time. In addition it's nice to be able to check my IAM, my timing, etc. without a laptop.
I like real time map switching (between my aggressive and street map - which soon will be between street and E85).
I love how I can change my launch control in a matter of seconds. With open source colin has to reflash a map every time he wants to change the launch control RPM. On track days before I pull into the staging area, I'll sit and play with my launch control and boost gauge to determine for that day what RPM is giving me the most boost during launch control. There's no way possible that with opensource I would be able to do this as quickly and efficiently.
It's also nice to have the check engine light reading and clearing. I've read CELs on other people's cars before with it too. My buddy's Jeep, and a friend's Toyota. When I'm in the middle of nowhere and I get a check engine light, it's much more convenient to scroll to the diagnostic's section and click read instead of pull out my laptop... boot my laptop, plug in the cable, load the program, read, and clear. it's a 5 minute process with the laptop, and a 15 second process with the accessport.
In addition I have RaceTUNER, and I'm starting to learn how to tune myself. It's the same exact thing as using opensource tuning software (romraider).
I can keep going for an hour on other reasons... but over all I believe that for a Newb it's the best way to go, and for someone who tracks the car, or is getting experienced, I definitely think it's the way to go.
The only reason not to get an Accessport? You want to convert from map to maf (still haven't confirmed if this is possible with AP), you have too much power for the stock ECU (so far a 32bit ECU hasn't been maxed out to my knowledge but sensors have), or you are a cheap bastard. The reason why most go with Opensource? Cheap bastards... lol