Lewis Hamilton calls the car "stunning", and McLaren's Jenson Button thinks it is also fast.
"I was with (Daniel) Ricciardo for several laps and he couldn't pass me on the straights," said the Briton at the end of pre-season testing in Bahrain.
"He passed me on the outside of turn 11, which is high speed -- I've never seen anything like that before," Button is quoted by Spain's Marca sports newspaper.
Red Bull's Dr Helmut Marko said the team's normal development processes are also working well ahead of the new season.
"Our measurements have shown that the new parts we were bringing worked as desired, making the car better," he is quoted by Germany's Auto Bild.
"The fact that it did not look good on the track was due to the engine software," he revealed.
Marko said Red Bull and Renault are working hard together in the remaining days until Melbourne.
"We have so much to do," he told Bild newspaper, "especially the guys responsible for the software who will now slave around the clock on 18 hour shifts.
"But whoever can win together, can also work hard together," Marko added.
Mercedes is the clear early-season title favourite, but the German squad's Toto Wolff is not writing off Red Bull.
"I am sure they will come back," he is quoted by the Frankfurter Allgemeine Zeitung newspaper. "The only question is when.
"In our view, hopefully as late as possible," Wolff is also quoted by APA news agency.
Those on the outside, however, are less sure Red Bull can recover quickly.
"I think a Mercedes will win the title," former F1 driver Timo Glock told T-online.
F1 chief executive Bernie Ecclestone is not unhappy his friend Sebastian Vettel and Red Bull are struggling, telling Bild newspaper the fans need a new winner to make the sport "exciting" again.
He added: "Red Bull are not going to run away just because they don't win everything again."