Well, i cannot say fascinated, since i've used it on home computer (fairly complex smartos environment with dozens of machines).

Currently i use Cacti, since i do not wish not to use agent software if
possible.
I just don't trust anyone
Check out the new Cacti, it's quite nice nowdays and even looks fancy.
SNMP is quite enough for 95% of monitoring. To bad is not widely accepted.

For else i have (for now) crontab with various stats* and plot.
This is for drill down analysis if required.

As for Prometheus, i actually love those kind of products you mention.
A lot of options and features, complex.

It's a kind of product you think upfront what you want, then fire and forget.

As for features - if we talk whole stack....

I like API - get graph with URL, inputting what you want directly into url.
When you draw a graph from gui it gives you a nice URL to use as well (or send around).

Language is not strange to me (GO), i personally like it and explored it a bit, should work on just any unix out there.

Alerting API is quite broad. From mails to endpoint API(s) to various stuff.

Last but not least, official client libs for java, ruby, go and scala.
Other unofficial in large numbers.
Include metrics in your apps is something i have yet to see from others
This can lower the debug time and/or expensive software such as dynatrace.

Since you are a ruby fan, take it for a spin

It's really fast with nginx infront(reverse proxy and internal load balancer), even on couple of AMD FM1 cores @ home, running that and dozens of smartos vms in virtualbox.

I'm willing to suggest this product to replace Cacti completely in my work environment, gradually for new projects.

I'm writing this as totally not affiliated with Prometheus or Cacti in any way.

Regards
Peasant.