The 2011 sandy bridge MBPs, especially the 15 and 17 inch ones seem to have a long lifespan, their CPUs are only marginally slower than the 2012 ivy bridge ones, they have thunderbolt, they have the latent capability to use 16GB RAM and potentially 32GB, they have SATA 6 controllers, they have decent graphics chips, they're quad core CPUs, only thing they really lack is USB 3.0, but there are workarounds for that. I could see my 2011 MBP 17 lasting me 6-7 years before I move onto something new.
The 2011 MBPs may be the most over-delivered notebook performance Apple has sold. Apple only shipped them with sata 3 hard drives, and 4GB of RAM, only promised 8GB RAM max, and it turns out they are actually able to handle much more ram and even faster sata 6 SSDs, plus with Thunderbolt, the expansion options are endless!