I subscribe to ExpertFlyer and find it very useful. Whether it is worth the money is another question, but $10 per month is low enough that I can just forget about it.
Obviously, the big advantage of this over a lot of other services is the award and upgrade buckets. Yes, you only get the upgrade buckets for DL, but this is pretty big, if you ask me. Also, you get award buckets for AF, NW (+ upgrades), OK, and AS, which can be a big help when looking to use miles.
I also really like the basic information. Yeah, I can get schedules elsewhere, but EF really presents the information in a really nice way. You can search across a full week of flights, and have a quick glance at fare bucket availability. The seat map function works pretty well. And it is nice to have the fare rules available (though the format can be hard to read). All in all, it is very useful. And the interface is very easy to use.