I highly rate Sealskinz products; it's just knowing when to use them. Due to their waterproofing rating – they're as good as a secondary barrier to water ingress, definitely not a primary barrier (depending on application).
I would say that using something in accordance with the socks [i.e. overshoes] is best for the heavy downpours many of us experience whilst out on our bikes. The socks themselves won't cover that sort of rain but gentle showers they'll be more than enough.
Generally speaking, to be fully protected against water, the product needs to have a waterproof rating of 10,000mm hydrostatic head (that's simply just the way that materials are measured against water ingress). The Sealskinz products are usually about 5,000mm. So, at best, they are highly water resistant at best.
However... A product only needs to be 1,000mm hydrostatic head to be sold as waterproof. It's just that there is a recommended level of waterproofing for the UK alone for complete water protection. Anything lower than 10,000mm and the product will begin leaking past it's protection level [depending on how highly it is rated].
Apologies for the really long answer; but it's the best way I could answer the question – as well as maybe stick up for Sealskinz in the process.