Adwords currently allows two ways to formats of advertising, image ads and text ads. Both are equally good and can definitely generate brand exposure. If your looking to build audience to your brand image ads are the best, you can make wise use of it by keeping low cost per views. If your trying to promote a blog, I think its best if you go to text ads. Google text ads are priced extremely low at $0.1 but there are chances that you may not appear on searches, but don't worry go ahead with $0.1 and keep increasing your bid every month.
I have launched many Google Adwords campaigns, but almost all just generated traffic not subscribers, but recently a friend of mine told me a simple trick, that is to send traffic to your welcome page instead of sending it all to your homepage. Your welcome page should contain a list of your best articles according to categories and above that give a short summary of your blog and encourage your visitors to subscribe. It is really better if you could give away a free ebook for subscribing. You can see Dumb Little Bloggers welcome page here.
If your planning to go ahead with image ads, develop a good catchy banners and logos, the 468*60 banners work the best.