Ludostone is probably the best item synergy you're likely to ever find, and the Dark Bum is amazing. Way better than Bum Friend, that's for sure.
Yeah, and I've seen other people who got an even stronger version when they got homing shots too. That looks even better... I'll just have to play a thousand times, I'll find it sooner or later.
I've gotten Mutant Spider + Brimstone before.
Charge time = forever, but you will kill nearly everything in a hit with a damage up or two.
I am to understand that synergistic tendencies + bums of darker variety = amazingness?
... yes.
Yeah, there are certain items that go VERY well together. Brimstone on its own isn't bad, but it's a base to work off. My best run got Polyphemus (mega tears with a very long delay and fairly short range) and Ludovico Technique (one giant tear that is controllable like Isaac is, but does constant damage scaling with damage stat), and another one had Mom's Contacts + Dark Matter + Brass Knuckles + a bunch of tears up pills + Anti-Gravity + Soy Milk + Scorpio, so I had stationary super-quickly created fearing shielded poison randomly higher damaging tears that I could release in a barrage at once.
Plus, Dark Bum is always amazing - he takes hearts that drop, and occasionally spawns either a hostile spider (smallest, basic kind), a friendly spider or most commonly a soul heart. So basically, you get extra HP containers if you're on a floor that gives you a lot of hearts.
Bum Friend is similar, but gives you bombs, keys, coins and (rarely) trinkets for coins it picks up.
I actually have a video of that Anti-Gravity run, though only post boss rush playing around with the tears.