My experience was that Lempiras and USD could be used interchangeably everywhere we went (Roatan, La Ceiba, Sambo Creek, Copan). Everywhere from street vendors to Sosa airlines would quote in both prices. It seemed my friend and I always had opposite currency, if I had $'s she had Lemps or vice versa... so we almost always got both quotes. Restaurant bills almost always in both currencies. The exchange rate was always 20:1, either direction, everyone seemed quite indifferent if they received either one.
It maybe worth noting that we never purchased anything of any significant value, so if we suffered a small loss in the exchange rate it wasn't worth calculating or worrying about.
Finally, don't worry if you end up at the airport when you leave with Lemps, the bank will exchange it back for USD. If you leave on a Sunday like we did, use the currency exchanger guy (his formal title!), we got almost exactly the bank rate. It is above board in case you were worried, the tourist police actually pointed him out to us when we asked about exchanging currency.
Have fun!