I am currently a customer of HyperSpin and Pingdom.
I have been a long time customer of HyperSpin now, and they have been fantastic! I am still getting their old pricing scheme which is great, would have no problem to pay their current pricing (top notch service!). I accidentally canceled my subscription (PayPal problem) and they let me re-subscribe at my old rate, very good customer service over there. SMS alerts have been spot on as well!
I recently heard about Pingdom through a promotion of theirs. They were offering one year of free monitoring so I jumped on it. Even at the price they charge for monitoring 5 websites at 1minute intervals it is a very good deal. Their monitoring is spot on as well. Only had their service for less then a month however.