To pile on with Gareth. I have tried it in different conditions in different races and in boat v boat practice. In these real world attempts, even in really light air, the boat gets better VMG with just the JIb/main in most cases. If there's any chop it's even worse with the gennaker. I will say, especially in light air, be aware of the wind direction. If there is a shift big enough to reach to the mark (maybe apparent wind up to about 15 degrees forward of the beam), yank the gennaker out and gain a few boat lengths on the fleet while the shift lasts. In light air you can point a little higher if you sit on the leeward side and hold the gennaker clew in tighter (no pumping). Play around with it and you'll start to get a sense of the limitations.

Reaching forward of the beam in heavier air is quite fun. You need to hike and move aft. But you're likely to have to fall off the wind to furl. You're DEFINITELY making better VMG upwind without the gennaker in these conditions.

I have seen some good sailors partially unfurl their gennaker on really light days. I haven't messed with this and can't speak to its efficacy.