people say banners don't work, don't listen to them.
especially if you have visual related products, you're missing out if you don't try the image ads, click to play video ads and the rich media template ads.
text ads are often nested with another (or 2 or 3 more) text ads in an adsense block... if your non-text ads perform well enough (laser target them!), they'll show and you get the whole adsense block to yourself!
go look at some pages where your content network ads are showing... see what you notice more there... text ads or banner image ads.
tip for you, put a face in the banner (if it fits with your product and message), the human face is a natural draw to browser's eyes.
and display the product as much as possible - that will lift your conversion rate. since you only pay for the clicks you get, make absolutely sure what you show in the ad is exactly what you offer on your site. skip the flashy, animated stuff - minimize your text in your images and present the product as best you can.
go to someplace like 99designs or designoutpost and hold a contest for your banner production, LOW cost and high quality and variety of input from graphic artists. once you have finalized your designs down to a few, check with others to see which ones have the best appeal and messaging for your audience. award several winners and rotate those banners to experiment.
adwords lists 2 banners sizes, the ones pre-crop and the sizes they'll crop to (post-crop) - for best visual results, produce your banners in the post-crop sizes so G doesn't edit your image and it looks as good as possible.
use as much of the 50kb size limit as possible - 50kb is still speedy! and the quality of your image is very important.
keep experimenting, some of the banners i've had success with were very different than my competitors banners.
have fun with it - there's TONS of cheap traffic available thru adwords because so many advertisers are doing a poor job of exploiting / maximizing this area.
[google.com...]
Also, to make sure you show on relevant sites, make sure your campaign has relevant keywords and also add placements where you would like your ad to show. This helps in ensuring Google shows your ads on similar sites across the network (rather than you wasting your money by showing up on not so great sites).
The display ad builder (link above) is useful for creating interactive ads (which can support multiple destination URLs).
If it's appropriate, local business ads and mobile ads also do well and don't take that long to create.
Text ads are your only option on search, so you have to live with them there. But rich media has a higher image recall, and higher CTRs than text ads (of course, measure conversion rates as well).