I liked 3.0 Magic Immunity.
I dislike how D&D magic doesn't get explained anywhere, and I prefer systems where magic flows from some comprehensible source. It's all fluff, but sometimes function follows form. If you explain how magic works, you can explain how it doesn't work. Thus the idea that something would be enormously resistant or immune to it can be explained, because once you understand the force driving something you can understand a forcing pushing against it.
The weird thing about 3.5 magic is that you don't really know what it is, so you have no fucking clue what it isn't. SR, immunities, globes of invulnerability, anti magic fields all make less sense when you don't know what they're cancelling or nullifying, and you don't know how they're doing it either.
My problem with 3.5 SR is that there are spells that get to ignore it. It's the same problem I have with 3.5 magic "immunity," I don't know why your magically conjured tentacles are any different from your magically conjured fire, and that's because they SHOULDN'T be different at all. It's a case of the rules being so awful that they interfere with the game. Ignoring SR should be exclusive to highly specialized spells. Everyone else should just have to fucking deal with it. It's not because I hate casters that I say this, it's because one technically viable caster just isn't as good as another because of how SR works. I am OK with ignoring SR, but not on entire schools of magic. Ignoring it should only happen when you're casting your "I pierce SR at the cost of weakened effect, but at least I don't have to deal with SR" spells.
Most of all the problem is that SR just has an extremely shitty name. You should not call it "spell" or "magic" resistance if it doesn't successfully resist either spells or other magic. Too many spells don't allow spell resistance for it to be called "spell resistance," and in fact there's another shitty word choice: "Allow" spell resistance. Like it needs permission to work.
As a CONCEPT I don't think I ever liked spell resistance. There are better ways to program greater resistance to magic. Crank their saves (you can put "vs magic" if you want), give them abilities like evasion and mettle, and allow them to force a save even when a spell doesn't allow it. Having another random statistic out there saying the same thing is annoying and just creates more die rolls, slowing the game down. It's better to improve on the mechanic that lets people feel responsible for their fates (Saves, because you get to roll your save) rather than make up another that just keeps one person rolling dice longer (Spell resistance, because the caster rolls). The more time you spend with everyone at the table engaged, the better off you'll be. Even if you end up with the same amount of dice rolls (which you do on spells that allow only a save or only SR), saves are still better because more people are playing at the same time.
