I've been webmastering seriously for about 2 years now. I've been a member of almost all of the known banner networks, and HitExchange is No 1. I still keep an eye on new exchanges, like Smartclicks, for example, but HitX still wins. Here are a few reasons...
The most impressive aspect to me, is that HitExchange does not show ads for themselves. LE and others take up to 50 percent of the impressions on their network for themselves. Hit X only shows member banners, so being a member feels more like being a member of a co-op, and not just an exchange.
GET WHAT YOU GIVE EXCHANGE RATIO
The reason Hit X is like a co-op is because when you send a visit to another member, your ratio increases. Unlike LE, which has a fixed 2:1 ratio, your Hit X ratio will increase if you put it in a good place, where people will see it, and then click it. The clicks increase your ratio. So, unlike LE, where clicks mean nothing, and placement is a joke, your banner won't be placed at the bottom of a page where no one will ever see it 1000 times and never get clicked. Hit X gives it's members a reason to put the banners in a good spot. This means not only the ratio, but the actual placement of Hit X is obviously going to be better than other exchanges.
You can have up to 10 banners, each one can be directed to any page on your website, and targeted to the categories that it relates to. This gives you the freedom not only to advertise your whole website, but special features, or specific parts of your website. Oh yeah, did I mention that Hit X allows 3 banner sizes?
The staff here is really cool. There is also an internal messaging system for you to use too. No more searching for customer service emails, and then not hearing back for a week. Hit X can be reached very easily via the member screen, and usually gets back to me within 2 or 3 business days.
I could probably write more, but I think you get the idea.
Hit X is the only banner exchange worth using.