Portal 2 is, in my mind, one of the best written games I've ever played. The character writing is amazing, the dialogue is quick and wonderful, and the world it weaves is FASCINATING even at the small glimpse of it we see.
Portal: Revolution easily steps into Portal 2's footsteps with the grace of a true Valve prequel (halfquel? midquel? it's between Portals 1 and 2) if the company had it in them to ever count to 3. It has all of the sardonic wit and clippiness of the original, while still carving out its own unique niche with new mechanics and puzzles. Its two characters (beside the traditional nonverbal player character, of course) are full of energy and are voice acted to the moon and back. If you had told me this was an officially produced prequel, I would believe you.
There's very little to hold it back other than the shorter runtime, which is understandable due to the nature of being made by a small team. The game bugged out and soft-locked me during the final fight (god bless games built in Source) and I would still rate it as highly as I did, even without a true conclusion.
If you liked Portals 1 and 2 and want more, this is more. Welcome back to Aperture.