I have owned the Flex 1500 for about six months as I write this. It takes a bit of getting used to. Over those six months, Flex have updated the software several times, and each time it has got better.
To be honest, several times I have thought about selling it, but now it has settled down and is a key part of my shack.
It is designed to be a good transverter driver and I use it to drive a 70MHz transverter. I have worked a few people on other bands, but 5 watts is not much power. I have used either the transverter output or the PA to drive various transverters, depending on the power requirements.
Under the original software it was not very stable, and often crashed. Now it is OK. I am using a 3.3GHz, twin core PC running XP with 3 GB of RAM. The thing that uses the computer resources is the display. I use the panadapter but if the core usage goes too high I turn off the display. Also, I find it best to disconnect the wi-fi on the PC to avoid unexpected demands on the processor. It will run on quite limited PCs if you turns off the panadaptor.
Tips, be patient, update the software regularly. Mine, oddly, will not start if the Flex1500 is turned on when I start the software. I leave it turned off until the software asks for it, and then I turn it on and select it once it from the list once it appears. No idea why, as this is the opposite of the recommended order of things.
It has been a bumpy ride, but now I like it.
Would I recommend it? Only if you have a good computer and only if you are willing to spend quite a bit of time getting it going. Also, be ready for it to crash from time to time.