My rather destructive son is actually really good with all things Wii. We got the WiiFit a couple of months after it came out after much hoping to find one in the stores and finally managing to stumble upon one. He loves it but gets quite frustrated with the tight rope walking one.
I like it but never used the yoga stuff other than to unlock everything because I need to complete things. For that I prefer something with an actual flow. It isn't bad for getting a better idea of the right balance for some of the poses though if you're new to them.
I rarely use it right now because I don't have time. Soon I'll probably start again in addition to my vinyasa yoga DVD (AKA I'm going to kick your ass and make you sweat for an hour or so.). I know other work outs are better but it makes things more fun and holds you somewhat accountable.