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Heroku is one of the oldest PaaS providers. The fact that it's been around for such a long time means that it had a lot of time to mature over the years. There's also a massive number of articles, guides and tutorials on Heroku out there for beginners and advanced users.
Pro Excellent error logs
When your deploy fails you see a legitimate error log. Many of the other PaaS give you nondescript messages and debugging is a pain. Debugging Heroku wins by comparison.
Pro Simple scaling
Heroku instances can easily be scaled up or down by increasing or decreasing the number of available dynos for that instance. This can be done through the CLI or through Heroku's web UI.
Pro Easy to start with
Getting started with Heroku is very easy. It's a very straightforward procedure and a beginner can set up their first app in two minutes. Often it's just a matter of a couple of
git commands and it's all set up and running. The official Heroku docmentation also helps a lot.
Pro Dedicated build servers
Heroku has dedicated servers for building app dependencies, to ensure that you won't have issues like "out of memory" errors when deploying your app.
Pro Add-ons let you easily add features and technologies
Heroku has a vast list of plugins and services that can be added to an instance. These plugins cover things from databases to email systems. This remove the task of having to install services and setting them up manually. Heroku does it all for you.
Pro Free option
Heroku offers a free tier which contains a single dyno instance. It offers 512MB of memory and 100MB swap space.
Con Really expensive
Heroku starts getting really expensive once you leave that free tier. It's not just the bare Heroku service that is costly, the addons as well are very pricey.
Con Further deployments are slow
While starting with Heroku is fast and easy, and the first few deployments are actually very fast, larger applications tend to have slower deployments. It takes some time for the dynos to restart and while they are restarting the application is completely offline. Which means that you can lose precious seconds of application time.
Con Constrained by addons
If you want to fully customize your production environment, then Heroku can be seriously constraining. Installing libraries or services can not be done unless there is already a Heroku plugin for it.