The Sector Imperials bases have been my default basing choice for a while now. I like them for being a quick and easy choice. They do have notable downsides, though.
More than anything, the cast is not crisp. The bases are cast in China and it looks like
GW went with the lowest bidder. This is something to be aware of in my opinion. I've come around to not minding because like I said, I want it quick and easy. For quality these things shouldn't be your first choice. Perhaps not even a consideration at all.
The bases are kind of flat with a few modestly raised details. In some cases you will want to work out which base to give to which model ahead of time so everyone in the squad stands level. It adds a little work and some leg poses on some bases just won't look good. There should be enough variety between base designs and poses to not make this an actual problem, though. And in the case of small things like rivets it's easy enough to trim them down.
GW only makes 25mm, 32mm and 40mm round bases. Some small ovals as well, I think. Large bases are supposed to use the matching detailing kit that has large, mostly flat panels you can add to a featureless base and then fill in the rest with sand. That should work fine. I have a box of these things but mostly keep away from large models, so I haven't tried them yet myself.
Example of a 25mm base:
