http://www.wired.com/reviews/2011/04/blackberry-playbook/
Wired. Pretty glowing review of the hardware and OS, but the stumbling blocks are a lack of native email, calendar, lack of apps and buggy beta performance of flash in the web browser.
To be honest, I really don't understand who uses native email clients anymore. I haven't used one for personal email since ten years ago. Personal choice, I know, but the native software is coming this summer, which is only a few months away. I'm sure third parties will jump on this as well.
Lack of apps is a problem, I guess, but the number of apps will grow as its released and people get their hands on it. Definitely is a downside compared to the more mature platforms.
Buggy flash in the browser is a problem. I'd rather have a website revert to a non-flash version, or have it tell me it won't load than have it crash. My guess is that the performance will be improved over time, but can they get it rock solid? The reviewer mentioned it improved even during the review period, so here's hoping. It looks like Ars completely disagrees on this one, and says the Flash experience is very very good, so I'm guessing they may have been testing a new version, or perhaps just different Flash content. You can turn flash off, altogether, which is nice, I guess.
Blackberry bridge is a nice feature that was untested. Obviously this is only beneficial to their current phone users.