What do you like best?
Honestly, I love the entire concept.
This is a great alternative to CSS3 and tracking all special individual Media Queries manually.
Rather than worrying about manually figuring and tracking if a device is mobile and being used as portrait or landscape, simply call the predefined variables (@portrait/@landscape) and you're set.
You can also create your own variables to help with your development, you can declare colors, widths, sizes or font-families and use the variables throughout your styling. If you need to change anything, just change the variables and you're set!
The overall styling of Bootstrap is a home-run too. The buttons are nicely rounded and come with 5 preset "statuses" so you can easily toggle color depending on the class.
What do you dislike?
Due to all the version updates and integrations across the web, it's a bit difficult to get this to play nice with Drupal. Sure, there's a module that you can install, but you still need to get into the core files and make sure your Bootstrap paths are correct, and 9 times out of 10, you'll need to go to Bootstraps main site, download a new file and override what Bootstrap initially gives you, after some time it works, and when it does, it's great, it's just the extra legwork that I don't like.
Lastly, If you want to do anything special with that mobile menu, get a cup of coffee and dive in. Editing that menu tends to be the hardest aspect for my installs. Something never triggers immediately right from a fresh install, and a bit of script editing and function calling needs to occur to get it running smoothly.
Recommendations to others considering the product
Though this is great for hitting the ground running.
If you're an avid Developer and don't want to add a better mobile experience to your projects without all the fluff this solution might provide, do a search for "CSS Flexbox" and enjoy!
It's becoming more browser compliant and development is much more light weight.
What business problems are you solving with the product? What benefits have you realized?
Easily integrating and transitioning static sites to be responsive.
I also use Bootstrap as my foundation for all my web design programming.