I've found it helpful on long haul flights; we can't afford business or first class out of pocket, but we do appreciate the extra leg room, free movies, and being served food first (or being offered the chance to purchase food first, on flights that don't provide a meal). For shorter haul flights, I haven't purchased, I don't think it is worth it. FWIW, the flight attendants also seem a bit nicer.

I believe Delta has also changed how they ticket Comfort Plus (at least domestically and certain int'l markets) - you have to buy at booking and can't upgrade after ticketing. I have an upcoming domestic flight where I can upgrade to first class for $100 but there's no CP option. We were in Europe a few weeks ago and were offered the chance to buy up pre-flight.

Only you can decide their "worth", but when the price is below about $40 I usually get them.
The seats are, supposedly, a little bigger or have more legroom. Often the upgrade includes a free drink as well.
In addition to the above you get "priority boarding", which you may get already if you carry the Delta Amex.
One thing I can tell you is that the cost of this upgrade can change daily or even multiple times per day. I know this from experience.
I booked a trip to Florida on Delta about 3 months ago. The trip isn't until late December. I occasionally check online to see if there are any deals on upgrades. The cost of the comfort seat has declined from over $100 to $26. As I tried to lock in the $26 seat I had multiple problems getting my iPad app to accept my CC payment. I switched from the app to the Delta web portal and the price had dropped to $22! All in the space of about 10 minutes.

They aren't too much more expensive than economy. Is it worth it? Does anyone have experience?