For the initial 20 minutes or so (I was watching it on a PS2 so I've really no idea how long it took) Alienator sets up an interesting premise. I don't think I've seen a slasher movie with an alien from another planet as the baddie before. However, interest soon turns into stunned disbelief as you realise the 'alien' is a huge body-builder woman in a steel bikini. Yes, Alienator is patently ridiculous.
Don't think I hold that against it. In the world of shlock-horror, patently ridiculous can often be a good sign. However, the blatant stupidity of its premise is all the movie really has going for it. Alienator is funny as hell, but it is also a shambolic suckfest of the highest order. Actors heap on failed attempts at seriousness, potentially genius lines of pure cheese dialogue are stumbled over with unnerving incompetence and the direction fails to sum up even one or two decent set-pieces. By the time the movie's finished you can barely see the original concept through the haystack of total tripe the team piled on it.
Add to this the fact that the 'Alien' just kills people by vaporising them, as opposed to doing any 'slashing' as such and you have a giant throbbing heap of good ideas being left to rot. You'll laugh at Alienator, but AT it, not with it. If that's your thing then go ahead and check it out.