It depends....
If you like the pro's and put up with the con's of a class/level system, don't want a particularly realistic or gritty system, and don't have a particular genre or setting in mind (that has another system designed for it) then a YES!
If however say you wanted to run a Buffy the Vampire Slayer game then D20 Modern's Shadow Chaser's is a poor imitation compared to the Buffy the Vampire Slayer RPG.
If you wanted to run a near future cyberpunk style game then either Cyberpunk or Shadowrun would probably be a better buy. D20 Modern has no cyberware as yet.
If you wanted to run a more distant future sci-fi space opera style game then D20 Traveller, D20 Fading Suns, D20 Star Wars or any number designed for the setting games would be better.
If you just want to dip you toe in the water of gaming in a modern setting or want to stick with a familiar 'heroic' system while building your own campaign world, style and setting then D20 Modern is for you.