The show definitely wasn't the same without Suzanne Somers. The whole situation surrounding her exit was handled lazily. The network didn't want to loose Somers because they saw her too important to the show, while the producers wanted her gone as soon as possible. They kept her around in the clunkiest way imaginable in my opinion.
I hate how Jenilee Harrison was done. They basically hired her to absorb some of the scripts they had already written with Somers in mind, but had no intention of keeping her full-time. I actually thought she did pretty good, even if her inexperience shined through occasionally. And they keep her for season six, add her to the intro, and then casually discard her towards the end of the year.
Priscilla Barnes was a good addition, even if I think she lacked the chemistry with John Ritter and Joyce DeWitt that Somers had. I don't find her as funny, either.
Don Knotts was hilarious, and he filled a vacancy Norman Fell and Audra Lindley had left, even if he filled it differently. I like both eras of landlords.
The biggest casting waste to me was Ann Wedgeworth. Her interpretation of Lana in the fourth season was hilarious, but the kinks with her and the writers couldn't get worked out, so she was gone by mid-season. It's a shame, too, because Wedgeworth is an attractive and talented actress.