So, for the record.
Novels written: 2
Query letters sent on first novel: 250+
Queries on first 'ready' draft of Aetherstorm (query and MS): 40
Requests on first 'ready' draft: 1
Queries on second 'ready' draft: 40
Requests on second 'ready' draft: 2
Queries on third 'ready' draft: 40
Requests on third 'ready' draft: 7 with 1 offer and 4 others still outstanding, I did not pursue offers from the other agents with outstanding material (but don't worry, I did inform them).
Lessons learned:
When you think it's ready, it's not ready. Don't rush into things. On my first manuscript I started to get some fairly serious attention at about the same time as I'd completely exhausted the pool of available agents. In the end you'll move more quickly by taking your time and getting it right.
Don't query overload. Querying can be addictive. It's like gambling, it's always tempting to just go for one more. Unlike gambling your chances improve over time as you make the work better, so save yourself until you are confident it's at the right level.
Keep plugging away at it. Find good beta readers. Don't let them take over, you have to decide what comments work and which don't, but don't ignore their advice either.
Come up with a good, unique high concept. The last full request I had on my first novel sent me a rejection which said it wasn't unique enough to stand out on the bookshelf. I really took that to heart, and I think that's what paid off in the end.