Good screen, great features, light weight yet durable. Plus since it is a Google device it has a much better chance of receiving most if not all future OS upgrades versus other tablet that you have to wait and see if the manufacturer will release updates.
Nexus 7 if you want a great Android experience with no work. If you don't mind flashing a new ROM, you can get a Kindle Fire or Nook Tablet used for about $100 that will work well too. If you want a 10" tablet, you can get a used iPad 1 for about $150 or a Tegra 2 tablet (there are quite a few to choose from) for about the same.
Nexus 7 if you want a 7"... otherwise an older generation iPad? Dunno if you can get the iPad 1 for that cost but if you could it'd be a good 10" choice (maybe).
I love my Blackberry Playbook, and it's great for browsing the web, but if you want to use apps then it's a poor choice (very little selection compared to android/apple)