Careful with the D20 modern character sheets if you want to make some editions to a game. Last time I tried that the PC's ended up being somewhat overpowered because some of the rules for the D20 modern classes weren't exactly something i had studied before (oops!). Other than that, D20 modern works quite well, classes/skills/feats are right on the money. Still, if anything, i always critizise the fact that the whole Hit Point based system is quite crappy. Example: Your character can be fighting with no penalties even after he's been stabbed, sliced, fletched, bitten, poked, burnt, and frozen, just as long as he still has 1 HP left... after that, he suddenly collapses on the floor and lays there bleeding. Quite... unrealistic... (this is not discussing fantasy gaming as "realistic", this is discussing game mechanics, so dont start going "but its a fantasy game, it doesnt have to be realistic" on me
). If anything, it should have some sort of penalty, at least for when you hit 10 HP when you're level 10 or whatnot... The only game that has overcome this is Star Wars, whose system i not only appreciate but encourage. Vitality and Wounds are a great idea...
Course, that's just my opinion...