I'm pretty heavily experienced with both the Cat6500 and 3750's.
With the 6500's, make sure you understand what features you want from it, and make sure that it supports them is the key - this is a hardware platform and certain features aren't fully supported (such as some netflow bits etc..etc..)
The 3750's have been pretty reliable in my experience, but I wouldn't advise putting more than a couple in a single stack as the master deals with the CPU processing (or did a while back) so in a large stack of say 4 - 7 switches you used to experience high utilisation of the CPU and lower performance.
I've not had any experience with the Juniper switches yet, but based on their routing platforms, I would expect them to do exactly what they say on the tin (unlike some of the Cisco range
)