On the surface, as a Rays-fan friend of mine told me immediately after the news broke: "It's a good feeling. Finally a lifetime Ray."
That has to be the initial reaction to this. Tampa Bay is an organization that is notoriously risk-averse, and they have given a lifetime contract to a star player. This deal shows just how highly they view Longoria, and how much Longoria enjoys playing for the Rays.
The extension will kick in at the same time the Rays will be able to renegotiate their TV deal, as Jonah Keri pointed out. Also, this contract could be beating not only inflation of the third base market, but the market as a whole. With the influx of new money from new TV deals across the league, teams will have more money to spend, and that should lead to the cost of a win increasing from where it currently sits (5-6 million per win).
On the baseball side, there isn't much not to like about Longoria. His career fWAR of 29.3 speaks for itself, and he is an excellent defender at third who can hit for power and draw walks. The important thing regarding Longoria's skill-set is that power and walks decline slower and less rapidly, meaning that even if Longoria does taper off at the end of the deal, that he should still provide some value.
Defensively he still provides a ton of value, with DRS having him down for 19, 20, and 22 runs saved in 2009-11 respectively. When running the bases, Longoria does not feature elite speed, but Fangraphs measures Longoria out to be worth roughly 9 runs to the Rays on the base-paths over his career.
Looking at it broadly, the Rays were able to lock in their franchise cornerstone at a market price now. Whether the deal turns out to be a bargain, or simply just a team paying market-value for a star, this deal looks good for Tampa Bay.