The answer to this question depends on what you are looking for in an antivirus product. Of course, free virus protection is better than no protection at all, but most worthwhile free products are designed to entice you to upgrade to the premium version. This usually means that you can only access the core antivirus engine, with all the heavyweight functions and features are reserved for premium products.
Premium versions also generally include extra layers of protection, such as web protection or anti-phishing technologies. Using free software, you are also likely to get bombarded with adverts for the premium version, which gets annoying after a while.