Dan,
I've been using Boost 1.51 for several projects now (the current version is 1.53) and it seems highly performant. I stick to those portions of boost that are header only (which seems to be the vast majority). I've heard people are wary about template bloat but I've never noticed it in either my own template code or in 3rd party library code, be it the STL or BOOST. I'm pretty sure the GCC linker optimizes away any code duplication. I mostly use the foreach and smart pointer classes in the library. I can see no reason not to use a highly tested compatible library like this. Two of the primary test compilers for BOOST are GCC: 4.1.2 and 4.2.4 (under linux), so I would think we're pretty safe with 4.2.1. I pretty much have eliminated all uses of new/delete and just use shared_ptr now.
Tod