I switched to True20 shortly after Blue Rose came out, and haven't looked back since.
The biggest hurdle was getting used to the damage track (as it seems to be for everyone). As soon as we had a couple of combats beneath our belt, however, it all fell into place. Now it's second nature.
My big wins:
I used to hate running NPC spellcasters, especially high-level ones. Now they're easy to stat and run. In fact, I can create spellcasters on the fly in True20.
Combat runs smoother, larger, and faster. The minions rule lets me run largish encounters quickly, while the PCs come out as heroes and villains stand a cut above their minions. They also use moves such as disarm, grapple, trip, and so forth far more readily. The Combat Unit template in the True20 Beastiary is absolutely inspired and perfect for handling followers, epic battles, and really big encounters.
I can improvise NPCs for combat or casting without statting in advance! Pick attack bonus, damage bonus, defense bonus, and toughness save. Attack and defence are level-based (and both based off same combat stat), damage and toughness are Str+weapon, Con+armour. Since I put weapon and armour on DM chart that means I can "reverse engineer" the stats later if I need to flesh out an improvised NPC. Skill checks and magic use are also level-based, so I can estimate numbers here if the NPC needs to make a check or cast a spell.
Now, you could do this with d20 if you knew the rules really well, but estimating HP is frankly harder than estimating Toughness (especially if you're trying to balance the encounter). Estimating skill ranks isn't hard, but good luck improvising a spellcaster. I was really bad at improvising NPCs for d20, but I actually ran an entire campaign without once fully statting out an NPC.
Easier to tinker. Sort of a side effect of having a lighter system, it's easier to focus attention on pet subsystems without overloading the GM. The crafting rules are very straightforward in comparison to d20 - great if you're not interested, and few/no dependencies if you want to swap in something more complicated. No alignment built into the system, so super-easy to attach your own complex moral code. Thanks to the simplified combat rules, you can even choose to de-emphasise combat (see Caliphate Nights as a great example of this!) in favour of, say, storytelling.
Less dependent on minis. Building on the last point, there's much less need for tactical awareness. In practice I've found that (my) players are more imaginative and engaged when they don't have to count squares or worry about optimizing iterative attacks.
Better customisation. You may need to provide heroic paths (sample level 1 characters) for your casual players, but the "feat every level" design means that people can come up with off-the-wall concepts and actually reflect that in their characters. I'm playing an illiterate dwarven skald who uses his shield as a percussion instrument. One of my companions is a greatclub-wielding giant mage. We're both first-level characters who already fit our character concepts.
Finally, the magic system is fantastic. Spells are much-more freeform and with a little experience a DM can make up DCs for any effect on the fly. Players use a bit more imagination since the spells are so much broader in scope.
) and I made almost every fatigue check. The other guy had bad luck, and was basically one and out. (We did have 'conviction' to burn to reverse it, but it didn't help him much.) So two characters of roughly similar abilities, one that was firing away and the other that was largely useless. 
