I've read several posts on this board stating that URC's wifi doesn't work very well (putting it politely). Does it have problems? Is this more of a install issue - poor wireless network configuration, too much interference, etc? Or is the design poor which makes their equipment very fussy to get to work properly?

I have several mx-5000's and 6000's out there with no problems. Just make sure your wifi network is bulletproof

How responsive is the MX-5000 when you're using it in WiFi? IE, pick it up after sitting for a half hour and attempt to do a wifi function such as change volume.

For regular commands I use rf so you do not have to wait for wifi. However, when I set the remote up I leave the wifi connected for 3 hours.

It takes about 8 to 10 seconds to reconnect to the network for my Integra processor.
I don't like leaving the Wifi connected for excessive periods of time, because it kills the battery life. If you leave the Wifi connected it becomes quite responsive.

My personal MX-5000 sits off of the charger for at least 8-10 hours per day with wifi set for 3 hours and I have never had an issue

There is always a trade-off when utilizing a wifi control system vs. non-wifi (zigbee, z-wave, proprietary, etc). One of the biggest disadvantages is the lag upon connection time that you may notice when you pick up your iphone or other wifi device. This is not URC, this is the nature of wifi and the time it takes to connect and verify credentials on the network.

If you can get past he lag upon connection, you will enjoy the ability to stream large blocks of data much faster and effectively. Items like cover art, and heavy imaging can come across like lightning. You will also notice a very good range output among other things. Again, the lag is the nature of the technology and is the main reason why people buy SONOS controllers vs. just use an iphone or itouch/ipad. The SONOS controller never drops the connection, so when you pick it up and wake it up it's ready to go.