We originally didn't have any kind of network monitoring software but after several outages that we didn't get alerted about until much later we decided we needed something.
Solarwinds Network Configuration Manager has saved our bacon several times whenever we needed to go back and compare config changes or when equipment has died and we had to go and find the configuration files again.
We considered Cacti and some of the open source products but they took to much time to set up for our large network to deal with. We ended up going with Solarwinds because of the simplicity of setting everything up.
After having Solarwinds NPM and NCM I couldn't imagine going back to life without it. Being able to get reports and alerts on various things is great.