Yes it's open to anybody, they are actually already selling it in a limited capacity, this new pre-order is for the wider general release. The reason for a pre-order system it's because they will limit the amount of people subscribing the first couple of years as they get more satellites up, from around 6k satellites up right now to around 40k.
It depends what you got right now. If something like Matty's numbers I wouldn't get it, if you got cable internet with around 100mb downloads speed I would stick with cable. If you only have DSL with something like 10 mb downloads yeah definitely. Like kato says rural or remote parts of town people will be the ones that benefit the most as they only get phone lines DSL.
What you got right now and how remote are you in town?