well, at IB (interactivebrokers.com),
i get better fills than my limit quite often...
because of the SMART Routing,
lets say you enter a limit buy order for Stock A at 23.55, now the order may first go to ARCA.
now the price comes down, and on ARCA it is 23.55 your bid / 23.56 ask.
now it happens that someone offers at 23.54 on Island or a different exchange.
so IB reroutes your order and buys those, so you get them for 23.54 although you had a 23.55 buy limit.
but that's only because of the SMART order (rerouting).
a limit order on the exchange always at limit (unless open/close as mentioned before)