There are two levels of lenses: consumer and professional. The consumer lenses are much more affordable, but you get what you pay for. If you're going to buy consumer-level lenses, buy Canon and nothing else. For example, Sigma's consumer-level lenses are not, IMO, worth the box in which they're packaged.
If you can afford them, buy professional-level lenses. They are build like tanks, use the best glass, and will last forever. And, yes, they are very expensive. Canon's L lenses are as good as they get. Sigma's EX series is also excellent and, in most cases, as good as Canon's L lenses. My collection is mostly L lenses. However, both of my macro lenses are Sigma EX, the 180 and the 50.
When it comes to camera equipment, the lenses are the investment. They are what you keep from camera body to camera body and they determine the quality of the image. The camera bodies, for all that we talk about them, are nothing more than a recording device. Use a cheap lens and you'll deliver a sub-standard image to the recording device. Use a quality lens and that same camera body will record an excellent image, assuming the organism that pushed the button did its job. ;)
Gary