"The Ruins" is neither a 1 nor a 10. A masterpiece it is not, but it's really not all that bad.
The thing that irks some people is that they want to know why stuff happens. In all honesty, it doesn't really matter here. Whether the characters know why or not, it won't do them any good in this case.
Personally, I sort of enjoy not knowing why. Much like "Cloverfield", the situation is what it is. It doesn't matter how the problem started; it only matters if/how you survive it.
I braced myself for a problematic ending from the get-go due to the reviews that I read before seeing the film. Yes, it's not a perfect ending, but it was not as bad as I expected. It could have been better and it also could have been worse.
Personally, I feel that "The Ruins" was better than the trailer presented it to be. It gave no indication why some girl's skin was crawling and, honestly, I didn't even care why it was. I was up for seeing a horror flick so I gave it a shot.
I'm happy enough with "The Ruins". Generally, it's an entertaining enough film.
If you don't have high expectations, enjoy horror and 90 minutes to kill, this movie might just be a good option for you.