link:
http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-January/012248.html
for what is worth when he said that a fully packed block composed exclusively of SegWit-enabled p2phk txs has an effective block size of 1.7MB it means that:
tx data + overhead + link to witness data + signature data (witness) = 1.7MB
those data are downloaded entirely by nodes that want to perform full validation, witness data could be discarded immidiately after though.
SegWit is a mere, although clever, reorg of the way we store tx data and hence block data.
A way that let you prune not only per block (e.g. keep only last 3000 blocks) but also intra-block, e.g. keep only the "message data" of the last 3000 blocks.
Other SegWit pros: all cases of non intentional malleability will be fixed, SPV clients will need to download less data, fraud proof if implemented will increase SPV client security, new script systems could be introduced easily. Of course SegWit will lessen full node storage requirement.
cons: Sergio's concerns about complexity that are amplified by rushed soft fork deployment. Garzik's complains about lack of economic analysis: