I've got the SnS plus... now they have the SnS 2.0. Not to debate the merits of those two, but I do wish I hadn't bought the plus 3-4 months ago when I started getting into this, because the 2.0 has some improved features. But that's not the point of this response. They're all great, in my opinion.
As most agree, the SnS is perhaps one of the best additions for a Weber kettle, as it has benefits in more than one area, is well made, and is (in my opinion) not gimmicky. It helps a Weber do what it does... better. I really don't think you'll regret the purchase if you make it, as it will last forever, can be used in all your kettles, etc. On the other hand, $130 is enough to buy 2 more used kettles on CL.
IF you don't want to smoke anything, or are already satisfied with 2 zone cooking through normal means, it might not be worth it. However, if you want to smoke something, or if you'd like to do 2 zone cooking as well as possible, it is great. (I don't use my WSM now unless I'm going 'big' on capacity. The kettle plus SnS is simpler, and uses less charcoal! 3 racks of ribs and under is a kettle + SnS for me.)
For smoking it REALLY helps regulate. It burns the charcoal more evenly/efficiently/slowly... and it insulates it from the walls. The water chamber is great as well. (But you can kinda do that with a pan.) Yes, you can smoke with the snake method... but not as well, and not with as much space. With the snake method, you've always got "some" direct heat applied to whatever you're cooking. Wherever the lit part is isn't THAT far away from your meat. If you're doing ribs, it will be close to the edges, etc. With the SnS, you have about 2/3 of the grill, and a nice divider that really keeps the direct heat away. You end up with more usable space, and a more controlled burn. The SnS REALLY shines for "slow" in my opinion.
It is good for searing, but IMHO... isn't worth it if better searing is your main goal. You can use your normal baskets (or whatever) to build a wall of sorts, and it works well enough. With the half of the grill sectioned off, you can get the job done. (And heck, you could buy a second or smaller kettle, and just get it rippin hot with a lot of charcoal. You can certainly sear the snot out of things in Webers without any additional hardware. WIth the SnS, however, you can do indirect better (better division) AND sear better with less charcoal. You can indeed get your coals closer to the grate (using less charcoal)... and still have a great indirect function. (Whereas if you're doing it with the 'normal' setup... just banking... to get it as hot, you'll overpower the rest of the kettle. BUT... the searing advantage is probably less pronounced.
So, in my opinion, if you want to smoke some, and have better 2 zone... AND be able to sear better, it is totally worth it. If you are JUST looking to sear better (and are not into smoking ribs, etc.), it is a lot to pay for improved searing.
I will point out, however... that YOU'RE ON A WEBER FORUM... so you really like your grill (enough to be wasting time on a website about it!)... and everything about your grill is good... and this is one of the best accessories... and it is going to be as long-lasting as your kettle (if not more)... and it makes it better in all aspects... and it works on both 22s and 26ers... and... you should probably just do it!