Pretty good until...

I sort of had the same problem. I didn't have super-high expectations but I was curious to see this after reading the short story and it was doing okay right up until the climax, which turned into a great big clichéd shootout in which everything explodes.

Now, I understand that an adaptation may need to make changes, and at first I was okay with some of the things it did. I was a tad bit annoyed that it took twenty minutes before anything from the story actually happened (the original short story simply begins with Jennings being led out by Rethrick after his memory has been wiped), but I was willing to forgive the film since it was taking the time to set up our hero and the world he lives in. I also didn't mind the fact that they updated things to the present day, after all a lot has happened since Phillip K. Dick wrote that story, and even the fact that the items in the envelope weren't exactly the same as in the story (for instance, the first item left by Jennings was a wire with which to pick a lock on a police vehicle while the officers were distracted, followed by a bus token with which he could hop on a passing bus and get away, while in the film it's a pack of cigarettes and glasses to see amidst all the smoke). I didn't even completely mind the differences in Uma Thurman's role from the female lead in the book.

But as you said the ending was just kind of a generic shoot-em-up action movie ending that you've seen a million times. I personally felt really annoyed since it was completely the opposite of how the original story ended. In the original work, Jennings sneaks into the plant alone, not with his girlfriend, and only wants to photograph the illegal time-scoop and steal the blueprints, which he turns over to his girlfriend for safekeeping, and then uses them to blackmail Rethrick into making him his partner, which works.

In the film there's all this stuff about the implications of such a machine (which was top-secret in the original because such technology was illegal, which was why the memory-wiping was used to begin with) and then Jennings decides the logical thing to do is destroy it. Naturally they break in very un-subtly (in the short story, he infiltrates the plant using another item he left himself that identifies him as a worker, sneaks around through an emergency shaft always remaining one step ahead of the guards, and finally gets out through a back exit using a pass key he also left himself). Personally I think the short story's climax was far more interesting, and nothing exploded there (and there was no biology lab battle, largely because there was no biology lab and the female lead was a secretary). Then of course in this version Rethrick gets shot and killed even though he (unwillingly) saved his life at the end of the short story (Rethrick wasn't actually pursuing Jennings in the original work, if anything it was more the other way around, and that was only because he couldn't prove to the police that he didn't know anything).

Of course, even discounting the fact that it's an adaptation I'm not quite sure the big shoot-em-up ending was really fitting for the rest of the plot, not to mention the happily ever after thing where Jennings and Rachel are working together in a flower shop (it was hinted that he might take over the company in the original story) and the FBI Agents spontaneously decide they no longer want to arrest him and claim he died in the blast.

And when Jack Crow and Buffy Summers joined forces, Edward Cullen never knew what hit him.