Honestly, I thought I'd be keeping the Magnet Sphere forever. Even with the slower movement speed, I didn't think differently about it.
But then I realized how powerful the Nebula Arcanum was. It was practically an upgrade minus the knockback: didn't dissipate upon contact with walls, dealt a much larger burst of damage, chased after enemies, and was faster. Once you've hid in a pit to maximize your chances of killing a Martian Saucer, and used them both, you realize that your Magnet Sphere simply can't compete.
Unfortunately... I think my Nebula Arcanum is well on it's way to being forgotten, thanks to the Last Prism. Every other magical weapon < Last Prism.
Actually, for that matter... I remember when the Rainbow Rod was the single most powerful magical weapon out there, and people shouted OP about it, just because with the right load out, one could do over 100 damage with it...