I bought a jailbroken PS Vita, and the first game I stole was TxK. By and large it's pretty awesome, very creative with the variety of enemies and levels you can get out of such rudimentary gameplay. The action is still legible despite all the trippy visual effects. It's hard enough that it always demands your full concentration, but it is a bit easier than 4K (I don't remember how hard 2K was). The soundtrack and sound design are fantastic, the sfx both punctuate the action and blend very well with the music. The best example of that is an enemy who gradually disintegrates, and does so to phone dialing sounds, something that in the context of the game's psychedelic techno almost sounds like another instrument.
The constant references to sheep and oxen were cute as fuck. Jeff Minter sounds like a real bro.
I have downloaded Jaguar and ps1 emulators to see how TxK holds up against other versions of Tempest. I own 4K but have not beaten it yet.