I don't know about you but I can't see a couple hundred miles out. I can with XM and that has made many flights more enjoyable.
I also have a really hard time seeing very far in solid IMC. XM is a hell of a lot better then nothing when you're in solid IMC with potential embedded activity. The key is to understand the weather system and the limitations of XM. I generally provide myself with a 40 mile buffer from anything significant if all I have is XM and I'm in IMC. It's much better then trusting that ATC will be watching out for you, because, often they do a **** job of that.
In most cases the delay of XM isn't a big deal. In the cases where it is you just need to increase the margins. I use XM to stay on the outside of nasty stuff. I don't try and penetrate to zig-zag my way through with it. Even if you do have radar, they can break, and I don't know many people flying with two of them.
That said there really isn't a case where having XM or not having XM will make the difference between a go/no-go decision for me. It just makes things a hell of a lot easier and certainly does provide some safety benefit. I'll take XM over SVT any day of the week.
I've never much cared for trying to listen to FSS or a controller describe the weather to me based on a bunch of fixes I've never heard of.