I have heard some really good things, and I have heard some meh things.
What are you looking for in a college?
CU Boulder is a good place to consider for a Colorado resident who can benefit from in-state tuition rates.
US News ranks its graduate-level physics programs #18 (which is pretty high).
It depends on what kind of math and physics student you are. If you are able to earn As you may be able to work at NIST in Boulder or JILA on campus. NIST is the National Institute of Standards, a lab of the US Department of Commerce that works on quantum computers, laser isolation of atoms, thin film superconductive devices (low temperature physics) and magnetics research. You can go on NIST’s website to understand what they do, be sure to look at NIST Boulder and not Gaithersburg MD labs. NIST is walking distance to CU Bouulders campus and does hire CU undergrads in a number of labs.
On the Boulder Campus, JILA is a very good physics lab, an NSF funded NIST/CU Boulder collaboration, where the Bose Einstein condensate was discovered. It may be quite difficult as an undergraduate to get to work with these scientists but some chance you could do that.
On the down side: Boulder is a big state school, with large class sizes, and a variety of instructors and professors in physics. Its going to take some persistance to get a lab position with a professor or at NIST Boulder, but the research level is top ranked. JILA is world renowned experimentalists and theorists , look over their website, but in a narrower specialty than say a physics program at MIT or Princeton or Stanford which has outstanding faculty in gravity, astrophysics, nuclear physics etc . Boulder and NIST have a special collaboration so there are NIST felllows on the Boulder campus who do not need to teach, which is why its so excellent, they are focused on physics.
Overall,as an undergrad experience, Boulder is not a lot of personal attention that you might get at a smaller physics program. but the research collaborations are very good with NIST. Also Engineering physics in the College of Engineering is a very strong math oriented degree. The level of physics courses is very rigorous at Boulder but a lot of undergrads drop out of physics. It is a large state school in a VERY distracting city offering many temptations.
Feel free to IM me privately if you have more questions. I worked for a time with NIST and very familiar with CU as well.