I've been looking for a well-made, high quality "XL" leather messenger bag for quite some time now.  I wanted something big enough to grow with (as I seem to accumulate more stuff that I Must Keep With Me! everyday) yet not a behemoth of a bag which would seem unwieldy.  I also didn't want to pay a lot of money for a low quality bag made of thin, shiny leather which feels like it would rip under the weight of all my items--but neither did I want a 15 lb giant of a bag.  When I stumbled upon Leyden & Sons I knew I'd hit the jackpot.
The bag arrived in two days, as ordered, and was packaged exceptionally well.  The box had a stylized, spray-painted LS on it when it arrived, and the packaging inside spared no attention to detail.  The bag was carefully wrapped in burlap and twine, nestled in some sort of straw--this is a very, very cool presentation.  This attention to detail on the packaging was echoed in the construction of the bag.
First of all, I got the XL version of the bag because I wanted the extra length in case I ever purchased a larger laptop.  It fits all my miscellaneous tech gear (including a Dell 15z in a padded sleeve), a cooling pad, power cords, other USB transfer cords, documents and notebooks, software, a hard drive, and more with ease.  The bag itself is made of thick (yet soft) leather and feels very sturdy indeed.  It's got a great look to it (very vintage) and I can already see it's going to age well.  As you use it, it acquires "scuffs" and marks which really make it look unique--not at all unattractive--in fact, it enhances the "vintage" look.  The rivets and stitching on the bag seem both well placed and sturdy.  All of the hardware echoes this quality--it's well made and very sturdy while maintaining a great look.  The inside is suede lined and has plenty of room.  The shoulder strap is awesome!! It has two wide leather pads on it which are great--the wideness makes the strap comfortable and distributes the weight of the bag more evenly rather than leaving a thinner leather strap to dig into your shoulder--very clever.
For quality, I'd give this bag a 5 out of 5--only time will tell how well it wears and survives daily use, but I'd bet it's going to perform fantastically.
For appearance, I'd give this bag a 5 out of 5.
For service, I'd give the company a 5 out of 5--they included a personalized, typed note which had an email address to use in case of issues, and when I emailed them during my ordering experience their reply was prompt, friendly, and helpful.
I'd buy this bag--or another one--from Leyden & Sons in a heartbeat.  I'm very impressed with their service and the quality of their goods, and look forward to years with this bag.

UPDATE: September 30, 2013: I've had this bag for just under a year now and it has worn in beautifully.  The leather has changed color to look more rich and "vintage", it has become more supple, and overall I love it.
The only con is that there is definitely some color transfer in the beginning with this bag, so avoid light colored clothing when wearing when you start using it. Also, avoid leaving paper bare in the bag without some sort of sleeve or folder--it too can get some color transfer on it.