The 3,1 was indeed one of, if not the, best value tower Apple ever made. Mine (dual 2.8's) has been in continuous use for about 9 years now, with upgraded graphics (280X), SSD's, etc. Its single-threaded performance is poor by today's standards (5 times slower than my 6700k Linux box running at 4.6 GHz). Its multi-core performance is disappointing too (about 3 times slower despite having twice the physical cores). The user interface can feel rather laggy compared to even my base MBP. However, the 3,1 doesn't break a sweat or even raise its fan speed no matter what I throw at it, so yes, it's still capable of heavy projects that run for days on end, and it still gets a lot of use.
I sometimes regret not buying a 5,1 when the last call for them came out. However, while the 3,1-5,1's were the best towers Apple ever made, I think all cMP's are now too obsolete to invest in anymore. Instead, my informal plan is to throw my trusty 3,1 a retirement party on it's 10th anniversary. If Apple delivers a worthy nnMP by then, great! Otherwise, the combination of the MBP and Linux will do.