Once the client is up and running it works fine on modern hardware. The new Notes 8 standard mail client (mail8.ntf) is on the slow side but I'm sure IBM is going to address this in 8.5.
Client startup times are a significant issue on older hardware, and I think it's going to be a major reason for people going with the basic client.
The other major show stopper is the lack of roaming (to be fixed in 8.5). Without it I know of no enterprise customers that could deploy the client in standard mode.
I guess we will have to wait until 8.5. Then we can let this baby out of the bag.