Not for use as your everyday charger. If you find you sometimes run out of time when charging your BlackBerry, it might be good to have as a quick top-up charger.
Whether or not that's worth it is up to you. Depending on your schedule, it might even be worth it to you to take the chance of prematurely replacing your battery for the convenience of very rapid charging.
You can but it will age your battery faster due to rapid charging and overheating.
Is this based on your personal experience? The instructions that come with the Playbook charger say nothing about this, and specifically say that you _can_ use it with your phone.
It is based on my knowledge of and past experience with lithium chemistry batteries. The maximum safe charge rate is 1 C, and even then the more preferred charge rate is 0.7 C. The OP's profile says he has the 9800 and will be picking up a 9900, both of which have around 1200 mAh batteries (1230 vs. 1250 mAh). What I have read about the charging circuits in these phones suggests that they do not regulate charge rate, rather they use the available current provided by the charger. The PlayBook rapid charger, at 2.0 A, provides a charge rate of 1.6 C.
A 1.2 A charger provides the maximum "safe" charge rate, and a 0.84 A charger provides the "preferred" 0.7 C rate.
