The 3D is a nice effect, and I keep it on all the time. In fact, the games themselves seem a bit odd when the 3D is off, since it loses that depth. That's just me though.
As was mentioned by yuyuyup, it's not the 3D effect that is losing excitement. It is the initial reaction. Back when the 3DS was first publicly displayed, a lot of people were excited and shocked because of the fact it was glasses-free 3D. Not a whole lot of devices like TVs had this, and this was being incorporated into a mass-marketed device. Now, the initial shock reaction is lower because there are more glasses-free devices released, so it now seems like the 3DS is just part of the crowd, even though it was one of the first to start it up.