I personally have a STX 12F and 15F... and for PWC's, they're fantastic. So far very reliable, low end grunt is good, acceleration is enough to enjoy, they are very stable (unless you do something really stupid of course), and mileage has been pretty decent. I think they're an all around great ski. The STX hulls have and still are being used for racing, because they handle well. Most guys will agree that Kawi hulls perform well in rough waters, and for the price, it's a real bargain. I've hit 55mph (gps) no problem w/ my stock 15F loaded up w/ a full tank, safety and fishing gear (maybe 3 rods, gaff, tackle, etc) out on the ocean. What I'm saying is when the fish aren't biting, or you're travelling to the spot... it's hella fun.
Space wise, it's not bad. I think Yamaha's gota bigger deck, but carefull planning can still set you up alright. I'm in the process of building a tank/rack for the STX... but currently use a PVC rack for the poles, and it's been fine (no live bait though). I usually invite a buddy and setup a milk crate w/ PVC pipes for rod holders, and a cargo net to secure stuff (kayak style), and that's been fine too.
Currently I'm using Humminbird Matrix 17 FF's (discontinued) mounted high on the right footwell. The screen is big, so this keeps it out of the way, and it's visible too. The STX doesn't have much room up near the dash. My Garmin Map76 is GPS is handlebar mount and works great. Oh, the transducer bracket for the Humminbird FF fits neatly into a small recess for the rear step ladder, so no cutting or modifying. Just zip tie and it's fine. I get clean readings up to 20+ mph (unless hopping off swells

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As for the 12F getting beat on swells... I understand, but it's kinda wierd. My 12F feels much lighter, and 'tingy' when it hits swells. In the air you can push down on the appropriate side to stabilize it, and it will respond. It definitely feels light, and 'hoppy' (but still stabil). The 15F on the other hand, feels much heavier for some reason. (it is a slightly heavier ski, but not by much). The 15F just powers through everything... rams through swells, and feels real solid. I think the weight difference is only about 40-50 pounds between the 12F and 15F, but I'd pick the 15F for rough waters... and for more fun and power.
The exterior design is getting dated, but personally, I still like it. It's a great combination of fun, reliability, speed, acceleration, stability, decent storage and practicality. There's many good and faster pwc's out there, but I do believe this is a well rounded ski.
If you dont care about blasting around, then a VX110 is a more practical ski... low price, better mpg, bigger rear deck... and the famouns Yamaha bait tank spout

I come across pretty decent deals on pwc's (yamaha's, honda, sea doo) but none have ever convinced me to give up my Kawi's.... yet.