I see alot of people asking what the difference is between DC Universe and Comixology Unlimited here it is:
Comixology Unlimited operates for the most part as a preview service. The idea is that you read a few trades worth of material to whet your appetite and a discount to encourage you to buy the rest of the series. When a series shows the Comixology Unlimited banner across the thumbnail and a number of issues that number is NOT the number included with your subscription. Take the current Aquaman series for instance. There are 53 books in this series at present. Of those 53 only 8 are part of Unlimited. Further those 8 include the first trade and its component issues. And don’t assume the other volumes will ever be on there or that you will care when they arrive if you read the first issue today. The first volume of SAGA has been on the service since its launched and they only added the second last month. Its still worth it as there is enough stuff that does stand alone and there is great stuff from other publishers too including Boom and Valiant who do use it as a true unlimites service.
DC Universe is much more committed to providing us with full series or story arcs. Everything up is part of the subscription and though there is a lot of further addition and/or subtraction that needs to be done to make the library fully readable its much easier to search. There is less new stuff here but there are more classic titles. It also has the video library, original series and the community boards which provide much better access to customer service channels.