From my perspective it has it's advantages and disadvantages. It's advantage is that allot of the coding such as the CSS is already pre-written and somewhat responsive and there are allot of nice looking themes already made that just require some editing. It's disadvantage from my perspective is in some cases it's faster to create websites from scratch. As you most likely will not find a pre-made theme that is W3C compliant and by the time you remove all the excess unnecessary code you could have been done already. To give you and example of what their CSS looks like just "Click Here". Quite a mess, but none the less it's has everything you can think of for it to splender.
Some of the nice features are all the pre-written snippets such as CSS buttons and progress bars and nice image effects it can let you do. If W3C is not a concern, than this a great route. Even if you buy a pre-written theme I would highly recommend converting the pages from html to php as most themes are saved in HTML format. This will make it much more flexible to start with and versatile in the future.
Bootstrap is great for new web designers to learn from and I highly recommend it after they go through their WordPress phase and want to learn coding using PHP and CSS.
An example of a nice looking theme I found can be seen here "Click Here". Keep in mind everything you see in this template theme I can make from scratch and W3C compliant. It really is that simple. If your hooked on Bootstrap that's ok as I can build using it without any hesitation.