It's great for seeing what pages look like, but not very good for showing you what the experience is like in those browsers. The big drawback, other than price, is that the experience varies from sluggish to extremely slow.
I do WordPress development and even with the VM setup this is a bit of a pain. Its ability to easily set up tunnels for local testing is excellent as well. Browserstack is great for quick and dirty testing, and is priceless for being able to quickly switch between different OS/browser combos.