I've only used Lambda for one thing, a simple website monitoring / alerting setup, but it worked wonderfully. I wrote my Lambda function in Java since I know Java better than the other options, and even in a verbose language like Java the actual code was probably 50-75 lines of meaningful code (not boilerplate). It just reads a file from S3 with a list of URL's, opens a connection to each in turn, and for any entry that experiences an error, it sends an SMS message to my phone using SNS. I probably spent an aggregate 3-4 hours on the whole thing, and it works great. And since I am only checking 4 times a day, it'll take me forever to use enough Lambda to cost anything.

It may not be perfect for all uses, but for this little thing it works out really well.

(Aside: Yes,I could have used an off-the-shelf package like Nagios, or a SaaS monitoring service. I did this in part to learn Lambda anyway, and also because I like the idea of something I control and can customize myself, but which doesn't require maintaining a server, yadda-yadda.)