IME: It really depends. It's not a silver bullet, you HAVE to measure and calculate [1]. The pricing is transparent, so it's more predictable.

My experience with single-function lambdas in JS was very positive, because they're quick to run and don't consume too much memory. They're also quicker to write and to publish, IMO. Some apps are virtually free to run.

Bloated-but-low-traffic enterprise apps are also a good fit, because the low traffic offsets the memory/execution time. I think Microsoft is in a great position here, because of Azure.

Customer facing app using an MVC framework? No way. Even Heroku can be 100x to 1000x cheaper.

[1] Here's a calculator: http://serverlesscalc.com