I think its fair assumption at this point that 8821's don't work well for a lot of organizations without a boat load of tinkering. To fill that void in our org we use yealink w52's and have about 200 bases deploying. I'm curious what other companies are using to supplement their wireless phone needs.
We had troubles with older models of 8821 as soon as we upgraded our wireless, since those old phone only really work well on 802.11a/b which SUCK!
After upgrading to 8821G it was pretty much plug and play and we do not have problems with them, but any wireless phone that supports SIP should be a feasible alternative.
Rolando A. Valenzuela
We've deployed it to almost every best practice in Cisco's guide and still cannot get it working 100%, I've talked to Cisco employees who say the 8821 doesn't work properly. When your standing in a single ap area its perfect but when you start roaming that's where the trouble comes in. We piloted zebra TC51/56's and those things work amazing, no drop outs when roaming and crystal clear voice with very little modification to our Cisco wireless environment, in that same pilot area the 8821 preform poorly. The zebra's are pretty expensive but it also provides a lot more solutions other than just operating as a voip phone.
As far as dect vs 8821 5/2.4ghz it is apples to oranges when it comes to technology but for the usability and reliability they should be equal and they are not.