As an old and new Swain player it's a mixed bag.
The biggest issue with the changes they made to Swain was that they really didn't modernize him; they started over from scratch. Some of the best reworks (Nunu and Warwick in particular) just modernized the way their kits work rather than rewriting them. With Swain they rewrote his kit and visuals from the ground up.
I think most Swain mains would have preferred something in the middle. A modernization without deleting what the old character was. We played old Swain because we LIKED old Swain. Whatever someone else thought of his visuals or kit we were having fun with it. Old Swain was also certainly no worse than a lot of other champions that were far more frustrating to deal with or still are in the case of champions like Malzahar or Vladamir, or new champions with triple dashes and invisibility.
Compared to old Swain who had high single target damage and solid crowd control mixed with the ability to heal tank when necessary the new one doesn't have a clearly defined strength. He is strong against divers, but any champion with crowd control and a defense mechanic shares that strength. He doesn't have the damage to be a burst champion, the amount of crowd control to truly control fights, or the amount of healing or other defensive steroid to be a real off tank (or have the base ratios or percentage damage somewhere). He does all of those things OK, but not better than anyone else. They played extremely safe when creating the new kit, frankly too safe, so that his strengths are not well pronounced. His Q especially now plays closer to Karthus, where you need to spam it to get damage in and hit the target (which is fine), but it also has a lot of gates on it including it's cooldown, the spread to hit all bolts (need to be at melee range), and the range in general vs artillery mages. Thankfully they are buffing it's range slightly next patch so maybe that will help.
In all fairness, Riot King Cobra has done a good job of interacting with the players and trying to add some more satisfaction to his kit this last patch. However, short of redoing the kit all together (something Cobra can't do all by himself), they've done the best they could in the mean time. I wouldn't be expecting a revert to the old kit anytime soon. IMO his visuals aren't perfect either. They missed a lot of opportunity with the transformation considering how much creative space there is to work with.
Gives you a real sense of what 
mains went through having their champions replaced except those champions have all had moments of being especially strong.
