FanFic A pretty good read.
Portal is kinda like a good topping. You just want to try a little bit of it on everything. So we've got it on My Little Pony Friendship Is Magic. How well do they go together? Good for the most part, but it might depend if you've got a preference one way or the other.
The fic has a number of good points to it, mainly in characterization. Chell and Derpy's personalities work well for their established characters without really contradicting canon too much, the new take on G La DOS is suitably threatening and no one suffers from any major Flanderization. The introduction of Portal elements into Equestria is at least within the realm of plausibility as well. The build-up and final conflict are also handled pretty well and make for a good action scene.
The downside, as mentioned before, is mostly how much you like your pony-to-Portal ratio. While it makes sense the final bits, set within Aperture Science, would focus on more on the Portal cast, the other ponies get pushed to the side fairly quickly, with more of their activities happening off-screen and their not even participating in the final fight. This may have been for pacing or simple lack of ideas on how to make it exciting, but it does undermine their role in the story at the end.
In the end, though, if you don't mind a little more Chell and GLaDOS focus at the end, you'll probably enjoy this story for the most part. It's written well and a solid read overall.
FanFic Simple But Solid And Entertaining
This fanfic is a good Portal fanfic in that aspect. While I loved Portal 2, part of the charm was of the original game was how much they did with so little.
The story takes a couple chapters to get going as Glados prepares to begin her beloved tests and Chell adapts to pony life. Where business picks up, of course, is once the testing begins, Glados is in prime form with her snark and humor as she breaks the heroes, and they're in-character too, Pinkie for example spending ten minutes in freefall between two portals for lulz. The final battle is strongly reminiscent of the bosses in the two games with Glados slowly losing control of the facility, but with an epic twist to the danger that cannot be spoiled. And the way she is slowly taken from power is simple, clever, and it works.
That really is the fic's appeal, I think. It's a simple story once it gets going - the ponies are subjected to Glados's tests and Chell works to save them and defeat Glados, and it tells that story well. It won't blow you away, but it will entertain you and hold your attention for a few hours.