I'm not exactly what I'd call a Veteran Savage Worlds GM. But I'd say maybe I'm Seasoned.
I've run several one shot sessions of SW, about 7 or so sessions of an ongoing fantasy game (that I'm adapting from a d20 Dungeon Crawl Classic module) for my wife and daughter and I'm about to wrap up a short (9 sessions I think) run of Deadlands: Reloaded with my regular gaming group.
Off the top of my head I'd first off say that Savage Worlds is, by its very nature, kind of "swingy". Certainly moreso than 4e D&D, which was the last major campaign that I ran. I don't regard swingy as a bad thing though. It's less predictable but that can be fun.
Balance is a tricky thing. In Deadlands they flat out tell you "Don't worry about it. If something is too hard for the PC's they need to learn to RUN." That's because the setting is literally supposed to be a scary place. But I still try and make at least a vague stab at balance.
In general I'd say that about 2 Extras per PC, if the stats on the Extra are in the same general ballpark as the PC's, is about right. One Extra per PC plus one Wild Card bad guy is another combo that is about right.
Bennies have a HUGE impact on this though. I ran an adventure a couple sessions ago in Deadlands where the PC's were being plagued by a Jackalope, which critters have the effect of bringing bad luck on the party, thus not allowing them to spend any Bennies until that gets resolved (often by killing the Jackalope). Man they were SCARED. I ambushed them ( a party of 4) with a group of 6 kind of wimpy Extras and they were worried the whole time because no Bennies means no Soak rolls and no blowing off a Shaken if you need to.
Make sure that both you and the players have a clear handle on how often you give out Bennies so they understand how free they should be with them. My general take on that is that I give extra Bennies when the PC's accomplish something pretty great (like surviving a really tough fight), when they bust out some really amazing roleplaying or when a player screws themself over because of their hindrances. On average I'd say I award about 1 or 2 extra Bennies per session than they start with.
As to the different genres of gaming question, I really feel like SW has become my go-to system for emulating a setting that I don't have a specific sourcebook for already. And sometimes even if I already have a book, I think that Savage Worlds does it better. For example, I'm not all that keen on the World of Darkness system from a mechanical standpoint. But I think a GM could take the fluff from the books, make a few minor rules changes, and run a really great game of Changeling or Mage.
Like I mentioned before, I'm using the Explorer's Edition by itself to run a fantasy game (though I'm planning to get the fantasy companion too) and I'll be running Space: 1889 using SW at GenCon in a couple weeks (you bet your ass I'll be picking THAT book up too though!).
Anyway bottom line is that I think it is a really great system and honestly I wish I'd discovered it YEARS before I really sank my teeth into it. I have Pepster to thank for that and he's somebody else who's input this thread would really benefit from.