I know there is a lot of lore already about Dona Tomas, but since my wife and I ate there this weekend, I must contribute my $0.02.
It is best not to wait until Saturday evening to make a reservation for Saturday evening at Dona Tomas. At about 5 o'clock we decided, based upon a spontaneous desire to eat huitlacoche, to go to D. Tomas, and our repeated attempts to call for reservations yielded only an unanswered phone. When we arrived, we were told that the wait for a party of two was 30 - 45 minutes. This proved accurate, and the hostess was unmoved by our report of the unanswered phone.
Fortunately, there were two open spots at the bar. The bartender was kind enough to mix me a Caipirinha based upon a vague description (I'd never had one before, only heard about it). Turns out they do a good volume in Cachaca over there, and I saw a great many mojitos go by, and maybe twice as many margaritas.
One overall note on the service - it seems that the popularity and success of the restaurant (in addition to who knows what) has created an atmosphere where the service staff "make the rules". I saw at least a couple of instances where customers didn't understand the unwritten code of the place, and were treated rudely. I don't understand the unwritten code of the place myself, but I'm so damn charming I didn't have any problems.
While we were sitting at the bar, we noticed that everyone but us had a plate of corn chips with crumbled cheese (cotijo?) and a ramekin of chile sauce. After enduring a few minutes of envy, we were seated at our dining table and our chips arrived. Not sure if this was the lagniappe du jour or what, but it presaged the whole meal. It said: "We understand chile." The chile sauce was a simple paste, probably of reconstuted dried guajillo, and the salty tang of the crumbling cheese brought the chile's promise to fruition.
Unfortunately, there was no huitlacoche on the menu. So, for appetizers, we ordered the Cocktail de Ostiones and Quesadillas de Hongos y Requeson. The first was similar to the Cocktail de Camarones which you can get at many Mexican restaurants (do yourself a favor and order it with the always surprisingly tender octopus [pulpo] when you get a chance), only this rendition was pervaded by the remarkably strong flavor of fresh oysters. Indeed, my wife was put off by the intensity of the oyster flavor, but I was intrigued by its boldness. This is the first oyster dish I've had anywhere that put actual oyster flavor so unabashedly in the foreground. The Quesdadillas were like folded 4" sopas de masa fresca, with a very edible balancing of earthy (morels), sweet and rich. Could have handled two orders of these.
With all the sampling, we chose to share and entree - Carnitas. The first time I had carnitas was in Jalisco, Mexico, at a roadside open-air joint full of cowboys, beer and live mariachi music. There, the carnitas consisted of entire pigs, hacked into managable pieces, cooking over a gas burner in a vessel that is the precise image of what comes to mind when you think of the word "cauldron". The pork sat in these cauldrons cooking in its own fat until you ordered some (giving guidelines as to the quantity and acceptable, um, "cuts"). Needless to say, the Dona Tomas version was quite different. In fact, what it reminded me of was Cuban pork roast, being apparently rubbed with oregano and other spices and dry roasted till it was crusty and pullable. I'd call it a bit dry, but the flavor was exceptional, and the white refritos that it came with provided good moisture.
Yes, dessert. Flan and pastel de chocolate. The flan was of the creme caramel variety - not the tamago-and-log-cabin-syrup loaf offered at so many other Mexican restaurants. Very delicate and smooth. The chocolate cake was characterized by a good balancing of chocolate flavors, as well as a background note of chile powder. The only thing that bummed me out was that the chile was not noted on the menu. Don't get me wrong, the chile-chocolate thing is all good, but I just wasn't in the mood for it. Like how I don't want chocolate with orange flavor when I'm in the mood for plain chocolate. They are just very different.
Anyhoo, another great part of the meal was the surprisingly reasonable price tag. A bit more expensive than a taqueria, but no more expensive than your average Thai or Indian meal.
So there you have it. In summary, call ahead (or be prepared to drink), brace yourself for some abuse, and order with faith in composition and execution and disregard for cost.