Exec Summary:  It's not the best deal, numerically, but it's close 
               if you change blades weekly.
 
Dollar Shave offers three types of razors (a cheap 2-blade, a decent 
4-blade, a deluxe 6-blade).  Dollar Shave will send you five 2-blades 
each month (for $3 each month), or four 4-blades each month (for $6 
each month), or four 6-blades each month (for $9 each month).  There 
is arguably a certain convenience in getting automatic monthly 
shipments (i.e., you don't have to keep a huge pile of spare blades 
sitting around), but I can't easily quantify that, and many companies 
offer simple one-click electronic order/shipment, so this doesn't 
seem a material factor in my analysis.
 
How long do you keep a razor blade?  Answers vary widely.  Once 
weekly, once every-two-weeks, and once monthly are the predominant 
patterns;  vendors advertise their blades as good for "for five to 
eight uses" (depending on thickness of your beard/hair/whatever), and
one supplier (whose name starts with a G) recently claimed that, with 
meticulous hot-water rinses, alcohol disinfectant, and maybe even 
denim stropping, a single razor blade could last as long as five 
weeks.  This isn't a one-size-fits-all situation.  I keep my razors 
for ~2 wks, which (for me) is approx. 6-to-8 shaves, and right at the 
end I can tell the blade is well and truly ready to be tossed.  
You'll have to jigger the numbers to fit your individual case.
 
Though I shaved with a 2-blade in my high-school and college years, 
I have since traded up to higher-end stuff, and it feels appreciably 
different, so I'll use the (middle tier) 4-blade for comparisons.  
The deep dark industry secret is that (most of) these razor-by-mail 
companies are selling you the same imported product (Dorco razors, 
originally manufactured in Korea), so we'll make this a six-way race:
Asian razors (sold by Dollar Shave), Asian razors (sold by ShaveMob), 
Asian razors (sold by Dorco USA), Schick Hydro (via Costco), Schick 
Hydro (via Walmart), and Gilette Fusion (via cheapest online source).
 
Dollar Shave = $72 for 4/month (free handle)  = 48 blades/yr for $72 **
ShaveMob = $40.98 for 28 blades (two handles) = 28 blades/yr for $41
Dorco USA = $31.90 for 26 blades (one handle) = 26 blades/yr for $32
 
Schick (Costco)  = $67.98 for 30 blades = 30 blades/yr for $68
Schick (Walmart) = $67.85 for 26 blades = 26 blades/yr for $68
 
Gilette (Google) = $76.55 for 28 blades  = 28 blades yearly for $77

 ** Dollar Shave Club has introduced an every-other-month plan
    since time of this review -- though this doesn't alter the
    value-per-blade numbers any, it DOES help with the medicine-
    cabinet-overflowing-with-surplus-razors situation.  It is 
    also possible to put your subscription on "pause" for a few
    weeks/months, thus limiting your excess-inventory further,
    but price-per-blade & price-per-shave remain ~1.50 and ~0.25.

From this it is clear that, though Dollar Shave isn't a "bad" deal, 
it is neither best per-blade nor per-yearly-batch.  Buyers do "best" 
with Dollar Shave if they discard razor-blades weekly, or if they use
the 4/month to stockpile a large supply, then discontinue the service 
and "coast" for a "free year" on their hoarded extras... but even in 
these use-cases, better prices can be had via competitor ShaveMob.com 
or via simply ordering Dorco direct.  For brand-loyal customers who 
absolutely MUST have a specific manufacturer (Gilette, Schick, etc.), 
Amazon or Walmart are best in retail segment.  (I've omitted the 
Costco yearly membership fee here;  it's not very much per visit, but 
might skew the comparison slightly toward Walmart.)
 
Does extra inventory on hand play into consideration?  (Bulk buyers 
will eventually end up with a few dozen spares boxed up in their 
closet(s), medicine cabinet(s), or sink(s).  Dorco USA's packaging 
is roughly tissue-box sized.)  That's not a big hassle for me, but 
it might be for some people.
 
Why did I even bother to do this analysis?  Firstly, when I get 
stressed out (over a job search, for instance), I tend to lapse into 
obsessive-compulsive behaviors.  But, more meaningfully, I was (still 
am) bombarded with those cute witty sarcastic Dollar Shave Club 
advertisements, and it got me looking into the company's particulars 
(founder bio, pricing, source(s)), and, while surfing around through 
said information, I found a fascinating blog post (on this subject) 
that referenced "mindless validation through consumerism."  And 
that mostly fits what's happening here.  The razors are not truly 
bargain-bottom priced (though they're not too far off).  The usage 
and inventory will (probably) not be in the amounts buyers need, 
without some tuning.  The original source product is, in fact, still 
available via parallel online channels at preferable pricing and
volume.  (Said vendor has issued a Web-commentary on this subject...
greatshavingripoff.com... which I submit without personal comment.)
 
What "value proposition" exists -- if any can truly be said to 
exist -- is "our diligent staff spend their work-shifts breaking 
apart large boxes of Asian razors and repackaging them into smaller 
boxes," and, possibly, "you will feel good about yourself when you 
click online, and when you receive your stylishly-packaged monthly 
shipment."  And you (the buyer) will pay somewhere between eight and 
forty extra dollars for that privilege.  I find that simultaneously 
fascinating, ingenious, and repugnant.