I like bob for practicing my accuracy, [but] heavy bags are better for conditioning and power.
Unless you fill the [BOB's] base with a lot of sand, it's pretty wobbly.
+1 (Sorry, I had to edit and punctuate... it's my thing.)
I used a BOB once when I was on vacation and visited a local gym. It really forced me to concentrate on accuracy -- at first I kept whiffing when attempting left hooks (my best punch, lol) to the chin. It was embarrassing.