I certainly would send an email, or even post a letter for more of an impact (no one writes on paper anymore, after all), to alert the restaurant to your experience. I am a firm believer that someone cannot correct their mistakes if they are not made aware of them. That is also the approach I use when complaining. I start out by saying I'm not complaining (which, actually, I am

), but bringing an issue to their attention which they might want to review and/or correct. I've needed to talk to a manager only two or three times in this manner (or, in one case, called their customer service number after stewing about my problem overnight) and have always received positive feedback. I talked to the manager at a local chain, that did have good food back in the day when we went out regularly, after we paid the bill. He compensated me by giving us a $50 gift card and invited me to let him know if we noticed a different the next time we were there. We gave them a month, went back - wow, what a difference! I'm pretty sure we weren't enjoying our meal more just because it ended up being free.
My best "get" was when we decided to revisit a nice restaurant when on vacation in Charleston, South Carolina. The first time we ate at
Poogan's Porch, the food was delicious but the server seemed to want to be anywhere but there. We decided to give them another chance and celebrate our 40th wedding anniversary there. I called ahead to speak with the manager, telling him that we loved the food but would like a more attentive server that night. Could he be sure we were seated where we would get proper attention? Not only did we have a wonderful server, the manager greeted us at our table with flutes of bubbly and a crab appetizer. Dessert was also on the house. By the time we increased what was already going to be a generous tip, we ended up paying for all of our "free" extras - and then some. Didn't matter - it was a wonderful night.
To balance things out, I also approach a manager to let them know if we thought our food was exceptional, or if we had a very attentive and efficient server. Leaving a generous tip for well-delivered service is one thing, but adding in a compliment to the restaurant owner can do wonders for that employee. I try to make sure I compliment at least as much as I complain. Mostly, I cook at home, though.