Yes, it was. When else have you ever seen a guy under pressure hold his serve 64 TIMES IN A ROW in the 5th set to stay in a match? You'll NEVER see that again, ever. Not even close. What Mahut did without mentally cracking for so many consecutive service games was inhuman.
And at no point during that match did I think Isner and Mahut had what it took to be a slam champion. It might be one of the most talked about matches, but in terms of quality you would have to be an absolute crack head to label that match as the greatest match of all time. Roddick's serving at Wimbledon 09 surpasses that, as he actually played three real returners (not Isner or Mahut) in Hewitt, Murray and Federer and got broken what, 5 times? In the final, considering fatigue and who he was playing, held serve until the very end.
Just because there is a bigger number to the score than some other epics (Fed v Safin AO 05 for example) does not make it a better match. You have more posts than anyone else in this forum, yet I wouldn't hold your opinion above someone like !tym, systematicanomoly, GuyClinch, Ollinger, Drakulie etc.