They had one very short period a couple months ago where they were slow-paying. I think they had a problem with their Neteller account. It lasted about a week as I recall and they bounced right back and, by all reports, have been model payers ever since.
My experience with IAS goes back maybe 6 months and has been very good. For baseball in particular they had good lines. I wish their limits were higher but they're not that kind of book so what can you do?