It's really unfortunate to see all the haterade being spilled over the comments. The efforts to take Bootstrap to version 3 were pretty significant, at the minimum they deserve a pat on the back for that alone.

Here are some important concepts I think a good portion of people are missing.

Upgrading - Don't upgrade for the sake of upgrading. If you have a fully functional site right now which uses or extends a previous version of Bootstrap your incentive to upgrade at this point is probably minimal.

At some point you had conceded that version X of Bootstrap was the right front end framework for your project. If that has changed you need now concede you have some work ahead of you.

Identify and evaluate the features introduced in Bootstrap 3 that are lacking from your current project. Scope out the markup and stylesheet refactoring required for any of the "must haves" and setup a schedule to integrate. Realize that this doesn't necessarily mean you are bringing Bootstrap 3 into your project, more so you may just be adapting your existing front end assets to model Bootstrap 3 features/patterns.

This is really no different than an existing site that doesn't use Bootstrap now wanting to bring the framework into their project. If you've identified that you need Bootstrap 3 for your project, you would expect there is work to be put in.

Usage - I see a number of comments critiquing design and usability. Is Bootstrap really intended to solve those problems for you?

In my opinion this is a framework intended to bootstrap your front end assets and development efforts. It prescribes conventions for common component markup and their respective selector naming conventions.

Think about that for a moment how valuable that is to your project (especially those with a large number of contributing developers). How many projects previous to one using Bootstrap did you have such well documented markup, style and naming conventions?

Bootstrap standardizes and documents front end patterns for rapid adoption and implementation into your project. That's pretty damn special. If what they promote doesn't align with your needs or development practices, well, don't use it.

Style - Bootstrap wasn't designed for your site, your users or even to keep up with the latest design trends. The styling, while in the view of many is decent, seems to have been intentionally minimalistic and not overly opinionated.

It provides basic structure and basic styling. It's your starting point. You can get to your destination now a bit quicker without needing to reset their style rules.

Having such minimalistic styles probably serves to be good motivation to get creative. If you want custom and don't have the design ability to get there, rest assured there will be a companion stylesheets to extend bootstrap coming soon to at a market place near you.

Inspire - If the Bootstrap shoe fits, wear it. If Foundation, Pure or any other front end framework fits the need, use it.

To me the great benefit of all these popular frameworks is they bring to light many well thought out development approaches to the front end. Not everyone will agree with them. They aren't always bullet proof. Though at the end of they day though they invoke developers to take a look at their individual assets and recognize areas of improvement.

Find the framework or approach that best aligns with your needs and personal preference and customize the hell out of it.