I bought a new K3 about a year and a half ago. I used it for several months. I then used an Icom 7600 for a while, and now I'm using the 5000A. I mostly chase DX on CW and SSB, and dabble in a few contests.
I think the K3 is a magnificent radio with customer support that is second to none. It has a huge list of features and a great subreceiver with a separate audio channel. However, the radio wasn't well suited to me for two reasons. First, I didn't like the quality of the received audio (to me, it has a harshness). After listening to it for several hours, I experienced "listener's fatigue". I used several different external speakers, and made many adjustments to the radio, but I was never satisfied. A lot of people share my opinion on this, but most owners are satisfied with the sound. Second, I didn't like the ergonomics. There were too many multi-use buttons that were difficult to remember, inconvenient to use, and they were too small for me to use comfortably.
The Icom 7600 had a great sound, a real nice display/panadapter, and better ergonomics than the K3. The 7600 has a dual watch feature to help with split operations, but both passbands are in the same audio channel. It's not as capable as the K3, but I found it more enjoyable to use.
I decided to try the 5000A, and I'm very glad that I did. It has a great sound that I never get tired of listening to. It also has two passbands in the basic radio, each in a separate audio channel which is really nice for split operations (in the same band). You can also get a subreceiver as an option, but I didn't feel that I needed it.
I had no idea how much I would come to value the 5000A panadapter display, the point-and-click tuning, and the ability to drag the filter skirts while watching them superimposed on the signal. As a result, I prefer the 5000A to any other radio that I've seen or used. I'm not saying that it's a better radio than the other top performers that are out there, just that I prefer it.
On the downside here are some negatives to consider:
1) A few users have some initial difficulties getting the radio to work with the computer (Flex personnel will help you if you have problems). For most, this is not an issue. I've had no problems.
2) Since part of the radio function is in the computer, you may experience some RF feedback problems into your transmit audio if your shack environment isn't clean. I had to install some chokes on my antenna feedlines and some ferrite beads on various cables in the shack to clean things up.
3) One of the benefits of having a software defined radio is that improvements can be made to the radio by simply downloading new software from the manufacturer. However, this only works to the extent that the manufacturer is committed to making the improvements. Although the Flex employees are very fine people and a pleasure to deal with, the improvements seem to come at a snail's pace (when compared for example to the electrifying pace of improvements/enhancements that Elecraft achieves with the K3). As a result, I have a concern that the 5000A may never become "all that it could be".
In conclusion, I'm really enjoying the Flex 5000A, and I hope that the Flex people continue to support and improve this fine radio.
Dan - W4TQ