The 50mm 1.8 (aka nifty fifty) is a superb lens for the money. Sure it is inexpensive so is made of cheaper materials that make it feel a little bit toy-like but the sharpness and quality of the photographs it can produce are very good, especially when compared to the kit 18-55 kit lens.
The 50mm 1.8 is a prime lens and generally speaking the quality of images produced by prime lenses is higher than that of zoom lenses, especially at the lower price point. The 50mm 1.8 will be far superior to the 50mm focal range of the 18-55 kit lens.
I spent about a year shooting with the nifty fifty almost exclusively and I think it is a really valuable tool with regard to improving your photography. I would recommend this approach to anyone interested in improving their technique.
I would agree with the comment posted in this thread that the 1.4 version is a better lens but it is also approximately three times the price and there is absolutely nothing wrong with the 1.8, especially if you are just starting out. The wide aperture will give you far more creative control and you will be extremely pleased with the results. It's a lot of fun.
I owned a Canon EF 70-300mm f/4-5.6 IS USM a few years ago which was a very reasonable lens for the price. Unfortunately from everything I have heard the Canon EF 75-300mm f/4-5.6 IS USM is generally considered to be a very poor lens, although I should make it clear that I have not used one personally.