I would say it depends on your iBook. I switched from a 800MHz laptop PIII PC to a PowerBook 1.0GHz and 512 MB Ram running VPC 6.0. In making my purchase decision I also considered a new 2.8GHz P4 laptop. At first I was very upset with my decision. Compared to the 2.8GHz machine the VPC emulation was soooo sloooow. However it was only a little bit slower than my old Laptop. Thus I was pissed at spending all the extra money on the Apple (it was about $1,000 more expensive) for a slower environment since it seems I spent ALL my time in the VPC. At the time I was doing VB, .NET, and MS Access.
After almost 2 years, I have come to the point where I would not trade my PowerBook for anything and am very happy with the decision I made. My change of heart comes from the fact that I now spend about 30% of my time in VB and MS Access. Thus I run the emulation for only 30% of my work, and can enjoy the apple features for the rest of the time. At 30% of the time, VPC is more than adequate. The IDE is snappy so you only notice performance issues when compiling or running queries. Large compiles and queries take a little longer but unless you are sitting next to a 3.0GHz PC you won't notice the difference.
If you are only using the PC environment 40% of the time or less, and you have a 1GHz G4 iBook or better I think you will be totally satisfied (just don't EVER compare it to a 3.0GHz PC running VB/Access or you will be sad).