I have dove Ginnie in late Jan to mid Feb on several occasions. It can get crowded on the weekends even that time of year so if it's possible, go on a weekday. If you have to go on a weekend and can get to the area on Fri evening, try to drop by and do all the paperwork and pay on Fri for the Sat dive. That way you can just stop by, pick up your passes and hit the water. We have gotten in one dive before most everyone else got through the checkin lines that way. We had the water before it got stirred up. Also, by getting there early, you can claim a good spot to put your gear. Take a couple of lights with you but be aware that unless you have a cave or cavern cert, the only place you will be allowed to take them is in the main spring.
The amount of thermal protection you need depends a lot on you , the water temp is around 72 deg. I typicaly use a 5mm long sleeve top, a fleece skin and a 1/8 hood. This seems to be plenty for me as long as I am actually diving. The coldest I have been there was a 50 deg rainy day one Feb. We were fine in the water but damn near froze to death when we got out.
Is it worth it? Well, it's a 10 hr drive for me but after 3 or 4 months of being cold and dry I am ready to go somewhere. It's a nice break once every 2 or 3 years, the other years I will head off to a different spring.