As Pettal as already said I've got an XS-600 (its actually featured on the XS-Ribs homepage:
and also in the owners comments section here:
Firstly the suggestion that they are unstable at rest. The tubes on my boat just touch the water at the back of the boat while at rest and unloaded, which provides a stable platform without sacrificing performance as they sit well clear of the water once underway. Once the boat has been loaded with a few bodies/dive gear it becomes even more stable as more of the tubes touch the water. The only situation I can see being a little unstable would be a on a very lightly loaded boat with a light engine. I've got a V6 Opti so that does make the boat sit lower and more stable at rest. With the 3 cylinder Opti (much lighter!) on a lightly loaded boat combined with the steep deep V of the XS then lateral stability may be an issue and there may be a see-saw effect. Pics of the boat loaded and unloaded are at the end of the reply.
You are spot on as regards value and quality. The performance, rough weather capability and general handling of our rig has been truly outstanding. I can just about squeeze 50knots out of our setup. Your budget of €18k will definitely get you a nice rig. Ours certainly cost a lot less than that €18k.
A few tips:
If you are getting an Optimax I recommend the V6 block ahead of the 3 cylinder as the added weight improves stability (see above). Also you do get a lot more for your extra money besides the added horsepower with a stainless prop and better twin smartcraft gauges instead of the single digital readout.
If you get a 2-stroke direct injection/carb bring back plenty of oil as its a hell of a lot cheaper in the UK I still have 12 gallons left since last May!!
Have a read off the following threads on the XS-600
and one on the exceptional service from Barnet Marine
If you have any questions don't hesitate to e-mail/pm me!! I'm only over the road in Carrigaline.