If you are looking for the best possible results, it will happen if you bet on a good design, a targeted message and at the same time a placement of appropriate sites. To a great extent, things depend on where you paid for your ad to be placed. The effect of a banner sometimes it's just for an image, but the good banner can also bring you direct sales. Usually a user clicks on an ad if he has already seen it on at least a few more places. Then his curiosity wakes up and he decides to check what it is all about. The effect is largely dependent on the message you are trying to convey. If the message is badly crafted, even if the design is great, it will be hard to get users to click.
There is a specific term called "banner blindness". It is used when users have already seen the same banner several times and have stopped paying attention. However, the process is inevitable. That's why most of the serious companies invest in banner design and new ads. This has to happen over a period of time. For example, you can order several banners at a time. So you can replace it if you need it.