Perot would probably be a disaster. Having said that, his budget ideas could gain traction with the limited government (dare I say libertarian?) wing of the Republican Party.
Because of this I think his presidency would be particularly painful for the Republican Party, because Perot is going to be proposing reforms that would meet what many limited government, no taxes Republicans dream of. Also, his opposition to NAFTA is going to draw many unions his way, potentially even many democrats. Perot might be turn his nationalist, small government backlash into a real political movement. Cutting taxes and putting more tariffs on Chinese imports, all from the same administration.
So while the GOP has its core coalition potentially splitting apart the Democrats get to unite themselves against this crazy man who is seeking to tear down the New Deal. No healthcare debate, the Welfare Reform packages far too radical to be taken seriously, constant media attention against various amateur mistakes the Perot Administration makes, '94 and '96 look like interesting elections that could go the Democrats way.
Anyway, Perot would fundamentally change the way that the '90s political battles went. I think he would gain enough traction with some Republicans to cripple the GOP for several election cycles, and even once he is gone, his election will have created a movement that will change the balance of power in the GOP. His strong anti-free trade sentiments in particular will serve to really reshape the way the GOP works.
On the Democratic side, I see a retrenchment of the New Deal-Great Soceity liberals in the party. Clinton's triangluation and Republican-lite moderation probably won't hold sway, especially while Perot provides a worst case scenario for Democrats to point to. The Democrats could also get pulled in an anti-trade direction, in an bid to make sure that unions don't jump ship. Mario Cuamo '96? I can see as a year when a Northeastern liberal would have a really good chance of getting the nomination.