I do weddings. What's a good wireless mic system to use for that?Answer:
I started my video production career working for a wedding videography company, so... I'm sorry ;)
But seriously... Here's a system I use and can personally vouch for:
You'll want to mic the groom and the bride as covertly as possible, but this is a great system if you want to be able to get a quality recording from a distance and still be able to move around with the camera. The only downside is that this system will cost $1,200-$1,400. If you're doing this kind of thing a lot, it's definitely worth it.
On the other hand... There are some less expensive options like this Shure BLX188 Dual Wireless System. Stuff like this is going to get you a quality recording, but it's designed to be attached to a stationary soundboard, and the receiver needs to be plugged into an outlet, so you'll either have to have your camera in a stationary place connected to the receiver, or you'll need to record the audio separately and re-sync it with your footage later on.
Another more affordable option I'd recommend if you're only recording the bride & groom (or, you know... bride & bride/groom & groom/etc.) when they're performing their vows, would be to rig a shotgun microphone with an XLR-based wireless transmitter above them hanging down, or on a short stand pointing upward between the couple. Audio Technica, Sony, Shure, Sennheiser, etc. all make one-channel transmitter/receivers for between $400-700.
Unfortunately, usually a boom operator is too intrusive at a wedding, or I'd recommend going with a shotgun and a boom run wirelessly through a single-channel system so that you could follow all that action at the reception as well without being limited only to the featured couple. But a shotgun mounted on the camera works pretty well too!
All that said, I will note that wireless systems can always be a risk as they can be disrupted by other radio signals. This is why I tend to prefer wired set-ups wherever possible.