Trimelli is located just across the Landstrasser Haupstrasse from the Mercure Biedermeier, where my wife and I recently had an eight-night stay. We found it handy and accommodating, and in the end had two lunches and a dinner there, all while sitting outdoors under the trees. During the hot weather Vienna was experiencing at the time, that sort of setting was most welcome.
The menu is straightforward Italian, offering pastas and *really* good pizzas at better-than-fair prices.Service was quite good, and we were able to get our orders across with a mix of broken German and even more erratic Italian (the fellow who had our table most of the time spoke English just about as well as we do, in fact, but he was very patient with us!). Best of all, in my view, was one of Austria's gifts to humanity -- Sekt vom Fass, or sparkling wine on tap. By the quarter- or half-liter it kept on flowing, and made our visits memorable indeed.
Like most Vienna restaurants, you'll find smokers here, and like most Trimelli does not accept American Express (note to AMEX: try lowering your fees a little and you'd do a lot more business in Austria). But both of those situations can be easily adapted to. Trimelli is a real asset in its neighborhood, as the large numbers of cheerful patrons we saw there reflected, and it added a great deal to our stay in Landstrasse.