Warriors ran in seven tries in the 50-20 win, roared on by a capacity crowd of 10,000 fans.
Speaking to Glasgow's official club website, Rennie said: "I thought we were excellent. The last ten minutes maybe weren't the best, but I don't want to speak negatively after a result like that.
"For large chunks of that game we were terrific. We were clinical in attack and turned defence into attack really well.
"It's perfect preparation for the final to be honest. If we'd won 50-3 or something it wouldn't have been as good for us - we were made to keep working right up until the final whistle.
"I think Ulster are a really good side, and we'd spoken about the need to be clinical with our opportunities before the game. Our set-piece was very good and we're the sort of side that can score in a blink, then score again in two blinks.
"I thought it was a pretty good overall performance, but we know we'll need to step it up again for the final."
The Warriors will make the short trip across town for next weekend's final against Leinster at Celtic Park - a fact that excites Rennie ahead of his side's first appearance in a final since their 2015 title win.
"Tonight was a big occasion for us, so I was rapt about the way we played," he said.
"We'd talked about wanting to be in a final at home in front of our friends and family - it's been our goal since the start of the season and we knew that if we got it right tonight we'd have that chance.
"The noise could be deafening next weekend. The crowd tonight were fantastic - the boys love it and to score first and go from there, when you're able to hear everything from the pitch, was brilliant."