I've had my server with Citadel for a couple months, and I have to say that they're absolutely astounding!
Their support is always fast, server's very responsive, and the ease of use when installing a load of mods is amazing!
Not to mention the price! I tried hosting elsewhere, Nitrado included, but encountered various issues that just made it.... bad.
I mean, yeah, they're not perfect, but what they do, they do really well, and for a reasonable price.
That being said; I highly suggest anyone looking to start a PZ server to try them out!