I've had one for a little more than a year. Can't compare with the Alpine as I've never used one, and it was about twice the price of the Predator.
I had to send the unit back to Predator as it overheated and melted the sheave carriers. Partially due to me being a bit careless about tightening the drive belt as I was breaking it in, and partially because, well, they were plastic carriers. Predator replaced the sheave carriers with metal ones. I've since developed the routine to regularly feel if the grinder head is getting warm. If it is overheating, it's a sign of the belt slipping and time to tighten the drive belt a little more. With the new bearing carriers, and proper belt tension, the unit has not given me much trouble since.
I haven't used it a lot, but it certainly has paid for itself and is great for those stumps that you just cannot access with heavier machinery. The largest stump I've done with it was probably 60-70 cm diameter, and really a bit too much for this small machine. But hey, it was good money and a good workout to boot
As others have mentioned, it really is important to keep those teeth sharp. Last time I ordered new teeth, they were GBP 9.95 each. The wheel is great for - well - wheeling the unit around. Keeping your left foot on the support may get a little awkward if the stump is on a slope, but is generally not a problem and it isn't always strictly necessary to keep your foot in place, even though Predator says so. The pivot point for the leg is just a clamp around the main shaft and can be easily adjusted. One of the few things that bugs me a little is the bayonet style mounting for the leg. The leg is mounted by angling the wheel all the way towards the engine, and when really reaching out with the grinder it may inadvertently come off. It really isn't more than a minor nuisance, though. The positive thing with the bayonet mount is that the unit is easy to put together without tools, and it has never really come out of control because of this and I have never felt that it has caused an unsafe situation.