The EF 75-300mm f/4-5.6 III USM can work as a low-cost first telephoto, but it has clear limitations. Community experience suggests it’s usable in good light and offers a useful telephoto range on the 550D, but optical quality is only mediocre compared with better telezooms. The slow f/4-5.6 aperture and lack of image stabilization make handheld shooting at 300mm harder, since you’ll often need higher shutter speeds to avoid blur. Some chromatic aberration is also expected.

The USM version should focus a bit faster and quieter than the non-USM version, but it doesn’t change the underlying optical performance much.

The most recommended alternative here is the Canon EF-S 55-250mm f/4-5.6 IS. It’s widely regarded as a better value on APS-C bodies like the 550D because it offers better image quality plus image stabilization, which is very helpful at longer focal lengths. Unless you specifically need compatibility with full-frame or film Canon bodies, the 55-250mm IS is the stronger budget choice.

So: the 75-300mm is acceptable if it’s very cheap and you mainly shoot in bright light, but the EF-S 55-250mm IS is the better buy for most 550D users.