What is most valuable?
The main characteristic I adore is the open source character of it. You want to monitor a specific metric that nobody else has? You can do it even with the most basic of scripting skills, and you can always share it with the vast community of Nagios Exchange.
How has it helped my organization?
Like any monitoring tool, it gave me insight into multiple infrastructures I've been a part of, without any limitation (due to the open-source character that I referred to above).
What needs improvement?
It's more what I personally don't like, rather than what areas need improvement. For example, the PNP4Nagios plugin not working easily with XI is an issue for me, because some open source monitoring plugins do not work out of the box. But in the end, you learn to live with it.
For how long have I used the solution?
I've been using Nagios since about 2005. I've seen the development path through the open-source version (and some other forks of it like Icinga and OMD) but for the last four years I've been entirely using the XI branch.
What do I think about the stability of the solution?
On older versions I had some minor issues. Currently, to be honest, it is as stable as I could hope for a monitoring tool.
What do I think about the scalability of the solution?
Up to now, the infrastructures I've been a part of were not so large, up to 200 hosts and 1300 services. Even for XI which uses MySQL on the back end, a host with 8GB RAM and four vCPUs is adequate.
How are customer service and technical support?
With the open source forks, the community is vast and so is the knowledge around the product. Because of this, even though I have a valid commercial support bundle, I have never had the need to use it.
Which solution did I use previously and why did I switch?
No, I started with Nagios. I've used other apps also like Microsoft SCOM (which is not very good), Zabbix (which is very decent), Tivoli (which is also not very good), HP OpenView (which is vast and requires almost a duplicate infrastructure to run to its full extent), Icinga (a very good clone), Centreon (haven't used it much but it seems solid enough), but I've always ended up using Nagios.
How was the initial setup?
For the latest versions, for me, it is pretty straightforward.
What's my experience with pricing, setup cost, and licensing?
For the cost of the commercial product and support, and taking into account the open source characteristics of it, I believe it is difficult to a better value. Yes, it needs some time to configure and address its issues, but seriously, which monitoring solution does not?
Which other solutions did I evaluate?
Before going to Nagios XI (commercial, meaning with support), because of the relationship my company had with Microsoft, I evaluated also SCOM. As with Nagios, I went through the whole installation and configuration process. Because of my previous knowledge, I directly compared it with Nagios, and the latter won, hands down.
What other advice do I have?
Be prepared to put some time into it and research it appropriately. If there is an option for consulting services through the support channel, don't be afraid to use it.