Thanks for asking - my agent sent out the ms to four publishing houses, with one-page descriptions of the other three proposed books in the series. The editors all had positive things to say, but one house made suggestions that really sparked something in me and I asked my agent if he minded another MAJOR rewrite to address the issues. His first response was to just leave it alone and continue submitting (those first 4 ultimately rejected), but then his internet went out for four hours and he had time to reflect and told me to go for it.
It was a little depressing to be back to rewriting (everything after page 60 was scrapped) after WAITING for that call. But here was the thing - as we were submitting, I re-read the first books in a ton of series in my genre (Sisterhood of Traveling Pants, Bras & Broomsticks, etc...) and realized that I was trying to tackle way too much for the first book. There was so much happeneing, it took away from the set up of the characters that is necessary for a series to take hold. That is partially because my book was originally a stand-alone that built into this great world with awesome characters that sort of asked to be continued. Once I 'saw' the weakness in that first ms, I couldn't help but to change it, even though I loved a lot about the original version (and can perhaps use some of it in the second book in the series).
When I finished my revisions, my agent liked them so much he contacted one of the rejecting editors who had loved my characters, but thought the plot was tackling too much, and she asked to take another look exclusively. Well, two and a half months later, we JUST got news that she's leaving the house, but has handed it off to another editor and so we are now waiting to hear all over again (but at least it is with someone with a totally fresh perspective).
So my advice would be to make sure your first book is super-strong, it needs to really grab attention, but should also be balanced with those character-driven elements that make series books appealing. Plus, read a ton of series - it is so interesting to see which ones make you want to run right out and get the next book in the series, and which ones leave you sort of 'meh.' You need to have your READERS first in your mind, then work backwards from there. My original query letter pitched the first book with a small mention at the end saying it was the first in a four-book series. But if I re-wrote that now, I'd probably be more up front about the other books. (Then again, that may not have gotten me as many requests) (or maybe it would've gotten me more!). So hard to tell in this biz.
Good luck with your submitting!