Not bad, not great, just kind of average
Lets start with the bad things. The game is short, really short. I already knew that it was gonna be short and its to be expected to be smaller then a full game, but I didn't expected it to be this short. In just around three hours I was through the main game and it really ended up feeling more like an elaborate tutorial then a proper full game.
The graphics are also not all that great and while my not-so-great graphics card is certainly getting some of the blame for that, I have seen better looking and more fluent games then this. The game also suffered from quite a bit of noticeable tearing, that even with vsync didn't want to completly go away. The main issue in the end however is really the graphic design itself, which is for most part just blunt and boring. Some of that might be needed to not distract from the puzzles, but still, a few more interesting things couldn't have hurt.
The controls are a bit of a mixed bag. For one thing I very much welcomed that the game can be played with a gamepad and my Xbox360 controller worked right out of the box without configuration. But the text-hints in the game still showed keyboard commands, not gamepad buttons. Not a big deal in a simple game as this, but still distracting. The controls itself also suffered a bit more getting stuck on wall-edges, there where quite a few situations in the game where something had to be executed fast and I ended up needing multiple tries since my character ended up hanging on an edge instead of moving forward. Edges of course become an even bigger issue when you carry something. The game also reorientates your character when it comes upside down out of a portal, which while necessary, just felt slow and distracting, making it hard to target anything directly after you got out of a portal. An added issue is that the game doesn't have clear visual indications for up and down, so its hard to guess in which way the auto-reorientation will rotate you ahead of time.
The actual puzzling in the game isn't bad, but neither all that great. Maybe thats due to the game already being a little older and me having seen 2D flash versions of the mechanics, tutorial videos and all that stuff that describes the mechanics that thus the novelty already wore off long ago. But overall I simply felt that the whole game could be completed with just a small handful of simple patterns and most of the time the execution was more tricky then figuring out the solution. Another issue with the puzzles is that most or them are just boring to solve, as you are forced to do lots and lots of waiting for platforms, energy balls and other moving things and there is simply nothing fun in waiting for a platform to be at the right position.
The worst part of the game is, as typical for a Valve game, yet again the story or lack there of. Some people might like it, but for me the whole speechless hero thing is just annoying. A story should be there to give motivation and reason behind your doing, but here it is, quite literally, just paint on the wall.
After all this trashing, the game of course also has its good points. The core Portal mechanic is fun and especially the last level is pretty nice. As that level breaks with the rigid nature of the other Portal puzzles and thus feels more like fun improvisation, then tedious executing of switch puzzles. The graphic style also changes for the better in that level.
Overall however Portal for me is kind of in the "meh"-territory. It is not bad, but neither all that remarkable. It has its one-trick-pony game mechanic that is fun for a while, but doesn't really go much beyond tutorial style gameplay with it. The last level opens things up for the better, but after that the game is already over.
