I've got it and it works just fine. The UI is a bit clunky (wish they'd used the default phone keyboard instead of coding their own, worse version) but it's fine once you get used to it, and it does the job it's supposed to do perfectly well.
No lag that i've noticed when navigating. In fact, it seems much more responsive than the iphone version (although that was running on a 3G, not a 3GS) - it was certainly MUCH quicker at getting a satellite lock than the iphone version.
Well worth the money IMO. Obviously android 2.0 is coming soon which will allow us to use google nav, but personally i like to know that i can use the satnav in the absence of a network connection. Also google already have quite enough of my data, thankyou. Not sure they need to know everywhere i drive to as well.
P.S. w.r.t. voices: You get a choice of about 8 installed voices (called things like luke, emily, henry etc, havent really played around with them much to know the differences). About 30 languages supported. Dont know if you can download 'novelty' voices or not, havent looked.