I'll caveat this post by saying that I'm NOT a crew-airplane bubba. My idea of a "monitored approach" is to fly a non-radar approach while a qualified radar controller on the ground monitors my alignment/glidepath to back me up in poor Wx/visibility. I'll comment from that perspective. If it's not what you need, disregard everything after "hello."
Some bases have a local reg that recommends monitoring (when available) when Wx is below a set minimum. I've done a couple of these, and it's no big deal...you just get handed off to a final controller freq (much like a PAR/ASR), check-in with the controller, and fly your approach.
The level of "controller intervention" has been a little different almost every time I've flown with monitoring. Some controllers will give you an advisory "approaching glidepath, wheels should be down" and others will pipe-up when you're a bit off the localizer. Other controllers will just STFU and only key the mic when they see you doing something that appears dangerous...and this is what I prefer (if I wanted a PAR, I would have asked for one).
Most of the time, I like the idea of a monitored approach. For a single-seat guy like me, it's one more guy in the loop who can QC what I'm doing and make sure I go home to my wife that night. If my nav gear goes tits-up inside the FAF, it's especially nice to have the controller ready to take over (if he's qualified).
Back when I was an IP at Sheppard, we'd have PAR-qualified controllers monitoring our solo students at night as they flew their ILS. Again...I think it was great to have another guy watching what was going on to make sure that unrated solo guys with less than 5 total night hours wouldn't smack their T-38s into the ground.