From what I've seen looking at recent reviews around everywhere regarding new GUI, I think those who've been continuously using Bittrex for the last few months do not like it (probably it's the same case with you), while those who gave up on Bittrex few months back and now just got back, do like it (my case).
I got used to the old user interface and this new one seems like its not as user friendly like the old one but I guess we are stuck with this new one and we just need to get used to it too like the old one. I thought I was logging in on a hacked Bittrex or a phishing site. The amount in my wallet took sometime before it shows the estimated amount. With the active markets shown in the left side of the screen, it makes me feel like i'm using Yobit exchange.
Yes, I do not agree with this sudden change of user interface from Bittrex. Bittrex should use polling to its users whether to use the old or new user interface so there will be no pros cons.
I also do not like it, because I think the old user interface is more Premium and for the new user interface I feel like a new exchange.