BizSpark consists primarily of two things: (1) a one-on-one contact to provide advice, to arrange partnership opportunities (marketing, etc), and to provide introductions to the Microsoft product teams as needed; and (2) free software licenses (which you get to keep at the end of the program). Loopt has also received other benefits, like access to their compatibility labs, conference passes, etc.
With BizSpark, Microsoft is confident enough in the quality of their offering that there's no commitment. You chose to use as much or as little of what is offered as you want, and there is zero pressure. We make use of various open source technologies[1]. It's not a problem. Also, at the end of the program they offer an extremely generous graduation offer[2] with no financial obligation.
After that, Microsoft's volume licensing is reasonable[3], the tools are well documented, and everything works as expected. Their stack is predictable, consistent, and dependable. If you want to be able to focus on your product instead of the tools you're using[4], then the Microsoft stack is great. Note: It's cheaper to have your tools work than to hire someone on salary to maintain and debug them.
[1] Ubuntu, PostgreSQL, MySQL, Ruby, Rails, etc.
[2] http://www.bizspark.com/Programs/Pages/GraduationOffer.aspx I can't discuss the details of Loopt's custom offer (we exceed the standard usage by a fair amount), but we'll be much better off after BizSpark than we were before. Microsoft has been very generous.
[3] See the SPLA program: http://www.microsoft.com/hosting/en/us/licensing/splabenefit...
[4] Not that they aren't extensible if needed. Back before ASP.Net MVC and WCF (and before Thrift, Protocol Buffers, and Rails) we wrote a ton of our own custom web and RPC tooling. ASP.Net is actually really nice but gets a bad rap because of web forms (which you don't have to use, and have improved substantially in the latest release). IIS 7 rocks as a transport agnostic application server, and can do MSMQ, TCP, UDP and more (it's extensible) in addition to HTTP.