The best way to explain how New Relic can help, is by example.
A few weeks back, one of the magento based webstes I manage had some serious delays at checkout. After 'submit payment' was clicked, some users reported that the site could take up to 40 seconds (and in some instances just timed out) to respond back with the resulting page. Yikes! This was just unaccepatble.
After spending nearly 5 hours tracking this down (without New Relic), and not getting a clear view of the reasons, and not being able to replicate the issue, I signed up to New Relic, and hooked the site up.
And within an hour I had the reasons, and a fixed was placed on the site.
New Relic has this great new feature called ' Key Transactions'. This feature gives you the ability to mark any transaction as important, and it will appear in a new menu called 'Transactions'. This feature makes it a lot easier to monitor the transactions you are specifically interested in. In my case, I marked the checkout process (/onestepcheckout/index/index) as a Key Transaction.
Ok, so now I can monitor the actual checkout process specifically, and I am ready to collect information on what the hell is going on.
I placed a test order. And yep, I could not replicate the issue, all looked good, and I got the result page within a few seconds.
But all was not lost. All I had to do was wait for New Relic to collect details about every checkout, and I coud analyze the results.
It did not take long to finally find the reason behind the intermitted delay.
The trace from New Relic for a transaction that took more than 20 seconds to complete:
as easy as that, I now had the reason for the delay: Email mailing list subscriptions at checkout!
The reason I could not replicate this issue was because my user was already subscribed to the mailing list, thus, at checkout I would never got the 'subcribe to mailing list' checkbox.
Now that I knew what was going on, I could easily relicate the issue, implement a fix, and eliminate the delay at checkout.
Without the help of New Relic, it would have been near impossible task to track this issue down. New Relic not only saved me hours of frustration, but saved me hours of time, and allowed me to quickly, effectively target, and fix the problem at hand.
Simply put: New Relic is a developers best friend.