Just got my #20 today & so far the biggest issue is the positive pin is too low for most of my RBAs, but works with about 75% of my tanks/Vivis, and EVODS & CE4s work fine with an adapter. I got one of the newer ones - it has the Stainless steel button and came with a Hot Spring already installed. Overall, it's a nice little MOD, I'll just have to fiddle with it a little to iron out some stuff I don't like (but I usually do that with most of my MODs anyway).
I've had my #19 for a few weeks, I hated it at first (a lot of miss-fires), but after some modifications, I love the thing.
First thing you HAVE to do is get a "Hot Spring" for a Silver Bullet and trash the stock stainless spring...hits 100% better, and feels far better also. Not a single miss-fire since I swapped the spring. You have to fiddle around with it to get the new spring to fit - I had to wind it tighter to fit smoothly inside the #19, but it's not hard to do. I would recommend NOT cutting it down if you retain the magnets - its a softer spring, and you need to have enough power in the spring to pull the magnet away from the battery. You also have to make sure the spring doesn't bind against the inside of the button if you want the "safety" to unscrew smoothly.
Some have said to remove the magnets, but I didn't find that necessary - I just polished them a bit and they work fine and don't appear to have much resistance at all... works great with them in. I also ground down the top of the 510 connector until it sat flush, hit it with a Scotch bright pad to even up all the machine marks and its good to go. After a few simple changes, it's become a new favorite (takes a Kick perfectly too BTW).
Sigelei has been coming out with some great little mechanicals lately...with a few simple tweaks, they are outstanding MODs. You can't beat them for the money....