First of all - I really like what Ethan has done - releasing the Nagios code as open source has really changed the way monitoring is being done today. For sure! It has triggered a lot of other projects, solutions, spinoffs, addons - and even businesses - in an unprecedented way. The guy should have a medal if you ask me
Having said that, I would really like to be able to say that XI is the best monitoring solution out there if you go the Nagios way. But unfortunately I can't. Maybe it depends on the size of the network you are monitoring but for us it just doesn't scale good enough.
Icinga, on the other hand, is in my opinion merely a feature bloat with a weird interface. They have some interesting ideas though - have to give them that - but I would put my money on op5 Monitor any day. Been using it for a couple of years and it just rocks! Beautiful and clean interface like nothing else and with merlin you can pretty much scale it anyway you want. (And I am NOT affiliated with op5 in any way - just love the product and what I can do with it).
Yes, it has a price tag - but so does XI and a lot of other solutions.
You get what you pay for - less problems.