We had a very similar situation ~300 writes per second on AWS. but I suspect some of this has to do with the fact that most people address scaling by adding a replica set, rather than the much hairier sharding setup (http://www.mongodb.org/display/DOCS/Sharding+Introduction), this seems natural b/c mongodb's 'scalability' is often touted. In reality though, because of the lock, RS dont really address the problem much, and we encountered many of the problems described by the OP.
Not to denigrate the work the 10gen guys are doing -- they are obviously working on a hard problem, and were very helpful, and the mms dashboard was nice to pinpoint issues.
We decided to switch too though in the end, though i still enjoy using mongo for small stuff here and there
Again ... this is more an indictment on the poor IO performance of Amazon EBS vs. MongoDB as a solution. MongoDB can scale both vertically and horizontally, but as with anything you scale on Amazon infrastructure, you are going to have to really think through strategies for dealing with the unpredictable performance of EBS. There are blog posts galore addressing this fact.
I often think MongoDB has suffered more as a young technology because of the proliferation of the AWS Cloud and the expectations of EBS performance.
In fact, I tried on-instance storage too -- this didnt help substantially. The reality is that many (most?) stacks these days need to be able live happily on AWS...
Not to denigrate the work the 10gen guys are doing -- they are obviously working on a hard problem, and were very helpful, and the mms dashboard was nice to pinpoint issues.
We decided to switch too though in the end, though i still enjoy using mongo for small stuff here and there