Thank God for cardtarget. If it wasn't for this website, I probably would have given up on etopps back in 2002 by selling my port within months of joining.
In a word, the site is an amazing tool to help you organize and evaluate your on-line portfolio and see how your cards performs not only in the last week but since the card was released at IPO with an easy-to-read bar graph. Rather than using the portfolio service for my in-ports, I've used it for my in-hands. Sure, it's a little time consuming to manually enter each card rather than use an XML file as with the in-ports. But it's well-worth the effort to help track your sales and (hopefully) your profits.
I like to use the "Watch" tracking tool to keep an eye on which cards I'm going after to have delivered. Right now, for example, I'm watching the 2007 Joba Chamberlain and the 2001 Albert Pujols to see how they will do as the baseball season nears.
Without CT, I would kick myself each time an autograph is offered if I didn't have the card in-port. With CT, I can buy the card immediately thanks to the CardTarget Market (or CTM) and get the card transferred to my portfolio within half an hour. Awhile ago, CT found a way to get CTM orders tranferred to their portfolios faster through the eTopps Trading Post.
Besides the CardTarget Market, the guys behind CT created the Partial Shares Market in which you can invest in professionally-graded vintage sports cards such as the 1968 Nolan Ryan rookie card graded PSA 9.
CardTarget also has been vital in providing information to the etopps community for each year's eCon, the annual etopps community convention that occurs at each year's National Sports Collectibles Convention (aka "the National).
There are also four different subscription services ranging from discounts on selling fees on CTM to unrestricted access to buyer/seller data on a per card basis.
The best thing about CardTarget is it's for free. Take advantage of it if you haven't yet.