Stackdriver is priced competitively and integrates very well with Google Cloud resources. It has a free uptime check tool which is nice (but not always reliable).
It is severely lacking in features and feels like a rudimentary beta product. We get what we pay for.
Also the UI is full of annoying bugs (e.g. when I mouse over dashboard settings the menu renders behind the dashboard, and once something breaks everything else on the page freezes and I have to reload often) - I hope Google fixes this soon.
I ended up moving to Datadog but continue to use Stackdriver for some less important GCP metrics that I can't export (without paying for the export, that is, since Datadog's GCP Integration calls to Google Cloud cost $$)
It's suitable for very basic monitoring and alerting but if you want anything beyond the basics, consider alternatives from dedicated metrics/monitoring companies like Datadog, New Relic, etc.
I use it exclusively for monitoring GCP resources.