I haven't always had the best of luck downloading free apps. Too many times I have had problems with bugs or annoying ads when I download a free app. Some of them may only give you partial features and you really need to upgrade if you want to get all of the features.
Because of this, almost all the apps I download from the store anymore are ones that I pay for. Most of them only cost a dollar or two, so it really isn't a big deal.