I never had any problems with payouts at SIA. My account was closed once I hit their threshold for winnings -- which is the norm for them -- but they paid my balance quickly.
Other SIA tidbits: The site is sometimes slow, especially during crunch time. The customer service isn't great though I've dealt with worse. I'm not a fan of their interface, but I've seen comments both good and bad in this regard for them.
What really distinguishes SIA from other books are their inflated prices for favorites. If you're a dog bettor/scalper/middler, you'll love them (though their limits are on the low side). If you find yourself mostly betting favorites, you should look elsewhere.
As for other books, I would recommend Pinnacle for their low vig, Olympic for their number of offerings (plus, they're often first to post lines), and Carib for just being a solid book in general along with quite a few daily perks/discounts. There's one other book I also really like, but since they don't advertise here, I'm not sure if it's appropriate to mention them.
Also, for whatever book(s) you choose, check their fees for payouts. Some are free; some are conditionally free; others charge a fee regardless. And, of course, method for payout factors into this.