I was part owner in Jabiru UL450 for a number of years and it shares a lot of features with the J400. My experience was similar to those who have already posted. The Jabiru engine in both 4 and 6 cylinder versions is a great piece of kit, provided one keeps up with the ADs. Same as any other engine really. Get familiar with what the importer, Skycraft say is required and you'll be fine.
Many will say that the Jab engines are somehow unreliable, but I'm certain that its simply not true. Operate them as the manual tells you to and they'll be more reliable for less fuel than the equivalent Conti/Lycoming. I've run mine at full throttle for extended periods of time (40+ minutes at way less than max RPM due to a very coarse prop) and it never missed a beat. I also crossed the Channel many times without fear of failure.
Also, the way to approach the landings is to keep a trickle of power in when you flare, as you can find yourself running out of elevator. After 10-20 landings like this you'll find yourself being able to flare without power.
The undercarriage legs appear to be consumable, and start to sag after about 500-800 hours, but they are not too expensive and are easy to replace yourself, its only three bolts on the UL 450, and probably is not too dissimilar on the J400. They give plenty of warning that they are worn out before they break. The gear is probably a weak point if you're used to being able to slam it down like a carrier pilot, but if you can land competently then its perfectly fine.
Go for it, you won't get a four seater that gives a better trade-off between speed, economy, purchase price and maintenance cost.