Q.U.B.E is what Portal might have been if it was about different coloured blocks instead of different coloured interdimensional doorways. Both take place in sterile test chambers lined with gleaming white tiles, and both involve solving lots of increasingly clever puzzles so you can eventually make a bid for freedom. The only real difference is that one gives you a giant gun to get the job done, while the other has a super cool pair of gloves.
I prefer Q.U.B.E's gloves. Yes, the Portal Gun would undoubtedly have a lot more practical applications outside of GLaDOS' hellscape, but the feeling you get from pushing and pulling bits of your environment around from across the other side of the room with just a flick of your finger is infinitely cooler in moment-to-moment puzzle solving than pulling a simple trigger. You're a Jedi in first-person space gloves, and nothing's going to stand in your way.
Q.U.B.E also does that brilliant thing all great puzzle games do - it builds and builds on its ideas until they reach their logical conclusion, riffing and iterating on certain actions or trains of thought without ever overstaying their welcome. Then it goes and introduces something completely new for you to get to grips with, before combining the two (or three, or four, or five) ideas into a series of mega-puzzles. It constantly keeps you on your toes, old Q.U.B.E, and by the end you get that same super genius feeling as its Valve-crafted rival.
Sure, the story isn't quite as twisty or as clever as it probably thinks it is, but that's not really what playing Q.U.B.E is about. It's about feeling like a wizard with telekinectic LED fingers. I highly recommend giving it a go if you get the chance (the swished up Director's Cut is less than a tenner on Steam at the time of writing) especially if you've got a Portal itch in desperate need of a good scratch.