How thoroughly has Segregated Witness been tested? I've encountered claims that Segregated Witness was "not really tested" and is being "rushed".

It is my understanding that SegWit had been running on Elements for more than half a year already, and that it is currently being run on its own testnet.


Has development of SegWit been rushed? 
What level of testing has been performed on Segregated Witness? 
Is it reasonable to expect SegWit to be sufficiently tested and ready for deployment in April?
I'd suggest starting here: https://bitcoincore.org/en/2015/12/23/capacity-increases-faq/. There's also a SegWit adoption page on the site somewhere.

Finally, this article has some details on recent testing: http://www.coindesk.com/bitcoin-scaling-segregated-witness-expected-launch/