I have an Optima Orca, which I love. 🙂
The criticisms above are all based on no/too little experience.
Invisible? Not so. Motorists can see the white lines, can’t they (ok, maybe not, judging by the lane discipline I see 😉 ). You are seen more because you’re different, and that draws attention. Anyway, there’re plenty of lower cars out there. I chose a high one not to be seen, but to see things myself – looking over bonnets at junctions, etc.
Unstable? Uncomfortable? That’s what a non-cyclist says about an upright when they first ride it. Balance takes a different skill set to riding a Diamond Frame, and you need to learn it. Likewise, although more natural, the position will still feel odd if you’re not used to it and might cause aches and pains at first – especially if you’re tense because you’re still developing the control skills. 🙂
Incidentally, I did once nod-off while riding mine across the Lincolnshire Fens… I was lucky not to end up in the dyke.
Not fast? Who cares? I ride mine for comfort, anyway, but a new rider will probably be slower than on a DF because of that different muscle-groups thing. They also climb slower, generally, because you can’t get out the saddle and honk. But that means you get to the top fresher, providing you have your recumbent legs.