I use on it's own and I could do 7 nights ok. But I'm a back sleeper and usually don't have too rocky of ground. For the 40 bucks, I say give it a shot and if it's not enough you can always cut it up for sitting pads.
+1 on the event bags. Always had a hard time justifying spending money on a bag inside a bag but it was worth it.
John, how are you using the SOL bivy in the pic above? Is it just for emergencies?