First off, the main difference between these and the ones from Secret Weapon is that these are plastic, so these can be attached to the base using model glue instead of super glue. They come in two halves with a beveled edge where they connect together.
So no need to worry about whether the two halves will fit together and need modification.
If you followed the link above you would have seen that they sell two types of adapter rings because not all 25mm bases are created equal apparently. This one, the "A" type, fits the GW bases really, really well.
It fits snug with no visible gaps. So far after having done a whole bags worth, they all fit the bases just like the picture above. A little liquid plastic glue and the adapter ring is locked on.
Here's a side by side comparison. Now, I will say that I have some of the Secret Weapon adapters that fit the base I put them on much like the Eccentric ring does. However, sadly, that was a minority of the ones I did. So yes, I am comparing it to one that shows how the majority of the resin rings turned out. As you can see, the resin ring has a gap between the base and the ring on the right side and the two halves don't fit all the way together (which may facilitate the gap).
Pros and cons
Secret Weapon Base AdaptersPros - I'm actually finding it hard to think of any right now.
Cons: More often than not it leaves a gap, sometimes sizable, between the base and the ring. The post and hole system just doesn't work well with either the hole full of resin or the post is too big, etc. Having to put them together with super glue, the only good way to do it is to have them sit on the table while you assemble them and you often end up with them glued to the table.
Eccentric AdaptersPros - Goes together with plastic glue. Fits snugly without gaps. Since they're in black plastic, if you paint your bases black like I do, there's no need to reprime models that are already done. Just add extra sand on the base, lock it down with thinned glue over top and then paint as normal and just leave the base black. No need to try to prime it with an already completed model.
Cons - Some of the ring halves on the sprue were actually attached to each other and have to be carefully taken apart.
Secret Weapon - $5 for 20 halves. ($0.50 per base for 10 bases)
Eccentric Miniatures - $3 for 24 halves. ($0.25 per base for 12 bases)
I will be sticking with the Eccentric bases. Better product (which I find difficult to believe as I have always found Secret Weapon products to be top notch) and at half the cost. I plan to make a sizable order from them in the near future. I have a crap ton of Marines and Necrons that need these. (okay, I know they don't NEED them, but I want them)