I also agree with BrianI and rfordut, as a 6'1" person who's not especially heavy, but certainly not thin, the synapse looks too small on me. I live in Seattle, so I've tried it on at the store multiple times. Everybody here seems to loooooovvvve their synapse, so I keep thinking "maybe it will look different this time", but alas. I'm eagerly awaiting the larger version, but for now I carry the SA every day I leave the house for work (mostly I telecommute). Also it is my general hold everything bag for doing things with my almost 3 year old. Remove the brain cell, add snacks, extra clothes, discarded coats when we get inside, water bottle, hat, umbrella, etc. But since I started waiting for the diaper bag before my wife actually got pregnant and we still haven't seen that, I don't think I'm going to hold my breath for the larger synapse. :-)
My SA is black/steel/solar, which I got not too long before they discontinued solar. It is the old style, and I've been tempted by the spiffy new features, but the lack of solar has so far been a deal breaker for me. If need be I can fit my personal 15"MBP in a brainCell, my work 15" Dell in a cache (got to have your priorities) plus both power supplies, cube with cables, mouse, usb drive, etc, JamBox, insulated KleanKanteen, umbrella (in pocket with KleanKanteen) pens, assorted bits and bobs, and still have a little room if I need to add something small. But when I don't stuff it it is even nicer to carry.
The upcoming larger Synapse is only 1 Liter smaller in capacity than the SA. So this really comes down to aesthetics, organization preference, fit and possibly cost.
The pricing of the new Synapse hasn't been announced but equipping the SA with pouches, modular pockets, etc. so you can organize small items in the same way can be costly. However, the advantage of a "bag in bag" organization design is that you can move the items from bag to bag easily, possibly use the pouches with other future Tom Bihn bags and removing them frees up a lot of storage capacity in the SA so you can customize it to meet various load out needs.
Also, if you're a bigger/taller person like me (I'm 6'4" 250 lbs), the larger Synapse's 20" length will probably fit your torso better. Don't get me wrong, the fit of the SA isn't bad. But, I have to loosen the pack straps a little more than what I'd normally prefer so the bag carries low enough to fall on my hips otherwise the waist belt becomes more of a abdomen belt. ;) Here is a pic of me wearing the SA:
You know, the size discussion does have me rethinking the SA now (I'm 6' and 200 lbs).
The "bag in bag" approach is also a very good point. I flip between my Empire Builder and my Brain Bag for work on a regular basis, and I have grown accustomed to keeping lots of organizer pouches handy for the "quick switch."
I just may need to buy both and try 'em out...
I own both the Synapse and the Smart Alec. I move back and forth between the two every few weeks depending on my needs. I like the organization of the Synapse even using a number of pouches and small sacks. I also love the basic openness of the Smart Alec which can be adapted to my needs. The Smart Alec seems to fit better - at at least feels better when wearing it. The Synapse has always felt a bit too small for me (I am 5ft 8.5 in - Lani is correct that extra 1/2 inch is important! - and weigh 165 lb). Unless I loosen the straps to the point that it feels like the straps will slip off my shoulders without using the chest strap (which I am not a fan of), the Synapse feels like it is riding a bit too high on my back. In contrast the Smart Alec falls just about at my low back/hips and feels like it will not slip off even when not using the chest strap. The one problem with my Smart Alex is that it is an older model with the flat/non-breatheable mesh back. My back sometimes gets more sweaty in warmer weather as a result (and it is often warm here in NC). The new Synapse (Synapse-grande) might be a good compromise...