The free Lite version is useful for many basic tasks, and smaller websites -- and some larger ones, I'm sure -- will be able to get by without paying. But the free tool is ultimately a brochure for the paying ones, and there are ample reasons why even a single-server site might choose to spend.
For instance, the Lite version records data from only the last two hours. If you want to know what happened last night or even while you were gone on a long lunch, you're out of luck. The full Standard version sucks down much more data about memory leaks, deadlocks, and other glitches. It also does a better job of presenting the information by grouping the servers into tiers and "learning" the behavior of the different methods and pages. For example, it will figure out that some distant call is always a bit slow and will stop flagging it with red every day.
Then there's the Cloud edition, named for AppDynamics' big target. The tool monitors instances of the application in virtual or cloud environments and helps with dynamic scaling in response to changes in demand. This will undoubtedly be where most of the development will come in the product over the next few years.
The main page for AppDynamics highlights failing requests and graphs the load and response times. Just drill down for more detail and diagnostics on slow and failed requests.
New Relic RPM Lite New Relic's telemetry begins just like AppDynamics with a JAR file tossed in the lib directory and a javaagent parameter added to the start command. The big difference is what you don't install: a monitoring server for digesting the data. This is all handled remotely on New Relic's servers. It's software as a service.
This sounded very inviting because I've wrestled with setting up a dashboard and server for Wily in the past. Letting someone else run this infrastructure was as good as it sounds. All of your account information is stuffed in the newrelic.yml file, and the statistics from your machines start appearing on your Web pages at newrelic.com.
This simplicity seemed less exciting, though, after I started up the AppDynamics stats daemon without any trouble or confusion. The only extra thought AppDynamics required was to type the command line starting up the daemon in the right sequence with the server -- which I had to try two or three times. Everything else was pretty automated, alleviating most of the advantages of leaving the stats collection to someone else. Both of these tools are dramatically easier to start up than what everyone endured even several years ago.