I have a SIP12" model. The bearings don't last long, BUT using higher quality ones (SKF, etc.) helps, as does using properly sealed ones. I don't think zz are properly sealed - they just have a cover over the actual ball race. I use ones with a rubber seal, and they seem to be good. They are cheap still and very common.
Also, make sure of your operating setup. My SIP needs regular fettling, and it is particularly important to set the guides up properly. Although I use a folded business card round the blade to get the spacing, one issue is that the saw's construction isn't very rigid, and the frame bends considerably when you tension big blades. This pulls the blade across to one side (out of square in both axes, as well!). It means you have to re-set the side bearings carefully, each time, otherwise one of them in each top or bottom set tends to rub a lot, whereas the other side barely touches. And yes, this knackers the 'busy' bearings. Part of the fix, which I really ought to do ASAP is to add extra welds to the frame, to make it more rigid
Those bearings are really there as a long stop (cricket simile). If the machine is correctly set up, for straight cuts the side bearings should play no part. If they are "needed" a lot during a cut then the machine's geometry is probably off.
I strongly recommend Steve Maskery's "Compleat* Bandsaw" DVDs - they originally gave me the understanding to do the setup properly (and make a good friend at the same time!). You have to do the right things in the right order to get it running sweetly. There are many how-tos on the internet about bandsaw setup. Most of them are either snake-oil, or made on high-quality big bandsaws, where things like frame distortion are not as big an issue.
It was necessary to modify my Sip (bottom wheel spindle) before I could properly align it, and even then it wasn't easy to get it right. Somehow it has slipped again (after a few years), and it's now bad enough to need another go. You will need a straightedge...
Worth doing some searches on this forum about bandsaw setup, but bear in mind my caveat about what's on YouTube, etc. There is one popular American who infuriates, as he pops up all over the place, giving the same advice at trade shows and on popular woodworking channels. Sadly it doesn't work unless your machine is pretty new and/or very well built, or you are only ever using small blades to cut curves.
Note to self: make some sharpie marks on the edge of the lower bearings to see more easily if they are rotating.
*After "The Compleat Angler" Izaak Walton, pub. 1653
