The nutritional content of tofu will vary somewhat according to the type of tofu. Firm has less water than soft, so it will be more nutrient-dense. A 4 oz serving (one quarter of the typical one pound package) contains about 80 calories, 9 grams protein, 2 grams carbohydrates, 5 grams fat, with significant amounts of iron and calcium. You can also find super-firm tofu, which packs around twice the protein--17 grams--of ordinary tofu.
The biggest downfall of tofu can also be considered it's greatest asset: lack of flavor. People often complain that tofu doesn't taste like anything. (C'mon--it tastes like tofu.) This gives you amazing opportunities to add tofu to a number of dishes where it can provide protein without altering the taste of the dish. I've thrown it in all sorts of things from pasta sauce (cut up into chunks) to vegan chocolate mousse (pureed smooth).
Don't be discouraged if the first tofu you buy is not as tasty as you want it to be. I've gone through many different brands and levels of firmness to find my favorites. The brands available for purchase at health food and specialty stores are definitely tastier than those at the general supermarket, and tend to cost about the same--less than $2 a pound here in Southern California. It is also possible to make tofu at home from soybeans, but that's definitely a topic to save for later.