Spirit looks to me like a strong competitor, but not threat to JetBlue, for several reasons.
First, they're aimed at different segments of the low-fare market. Spirit does a lot more charter, casino, and travel-agent work; JetBlue is aimed at more general leisure travelers and offers a wide range of connections at one hub. This, I think, is why JB offers multiple nonstops--eg four daily A320s to Rochester, NY--on its routes. It's using AirTran's hub-and-spoke strategy.
Spirit also links large northern markets directly to Florida, and does not serve medium-sized or small cities (little Atlantic City fits its casino-special niche). JetBlue offers fare relief to Rochester, Buffalo, Burlington, and has Syracuse, Albany and other medium-sized cities in its plans. Feed the pax to JFK, redistribute them to a wide variety of markets. A plan that would not work at LaGuardia.
Both airlines' strategies are good ones, and if their respective managements can keep costs and expansions in line with their capital, both should look forward to prosperous futures. In the New York market proper, Spirit has the edge to Florida by using LaGuardia. But with JetBlue's feed that may not be a problem.
And yes, it's just fine to use old planes! Valujet was framed by the media. The 1996 crash had nothing to do with the aircraft, it was an improper load that burned through control lines. A problem that was easily corrected. The story of how Big Air manipulated the media to libel and smear low-fare carriers with old planes, after that crash, is a Pulitzer Prize waiting to be won. We all know that planes maintained properly can fly for decades, as Northwest's DC-9s are doing today. :+)
I'd fly a Spirit DC9 tomorrow if I was going somewhere were they flew. And whoever suggested taking in old AA MD80's, that's a good idea too. From those big cities, Spirit should have no trouble filling the bigger plane.
Need a new airline paint scheme? Better call Saul! (Bass that is)