One negative is that they tend to dry out faster then plastic pots do. If you would like the look of clay and the advantages of plastic, buy plastic pots and put them inside your clay pots as liners. I use new soil every other year and compost all the old soil to be used in my other gardens. Almost anything will do well in containers. The few failures I did have were vegetables that didn't have a long enough season to mature.
Annual flowers are easy and beautiful container plants.
The key to having attractive containers, is putting the plants closer together and fertilizing and watering them more often. I hate to admit it, but I do use a water-soluble chemical fertilizer. I never use them on my other gardens because I use so much compost and other organic fertilizers. The problem with pots, is that every time you water them, some nutrients wash out. I use the fertilizer at one-half strength about once a week, and all my flowers do great.