I am happy to speak to this question since I am the treatment
coordinator in a SF Bay Area ortho practice.

1.  Align Technology is a corporation set up to make a profit.  They
spend lots of money on advertising.   The hefty fees for the computer
generated aligners is billed to your orthodontist up front.

To be candid, we have been put off by  their collection tactics that
are aggressive and not nearly as generous or lenient as the financial
arrangements you will find in most ortho practices.

Recently there have been competitors offering the same type of
technology.  Invisalign treatments, or treatments with clear aligners
might come down a bit in price as a result.  We hope so.

2.  Many people come in to our practice for initial evaluations hoping
for clear aligner treatments only to be told that their teeth are too
malaligned or the malocclusion too severe for the aligners to correct
satisfactorily.  They'll usually opt for traditional methods but
they're often disappointed.

3. General dentists can and do handle some of the easier corrections
with aligners but should problems be encountered later on in treatment,
some dentists simply find that they are uncertain of the course to
take.  You'll probably be better off searching for an orthodontist with
a good track record from the outset.

4.  Read the TIL contract carefully.  If there is no provision for
extended treatment, be sure to ask about that.  It is not uncommon for
people to be unhappy that their teeth have not reached the
"perfection'" that they had hoped for.  Most orthodontists will discuss
that in the pre-treatment consultation.  Our Dr. not only advises pts
from the outset that invisalign sometimes can not achieve as precise a
correction as traditional methods, but he also quotes the fee for
extended care in case the pt. opts for it down the line. Extended care
is usually a short period with traditional braces for finishing the
treatment.

5.  Orthodontics is not an exact science. It is tough to predict
outcome and it is tough to predict treatment time.  So much of it
depends on pt. cooperation, but some is simply nature taking its
course.
Our Dr. tells us that before attending his residency Ortho training he
was a general dentist for 5 years.  He attended courses in ortho back
then  and thought "Hey, I can do this."  It wasn't til later on, when
he was in the ortho specialty training program  that he realized how
little he really knew.  He has helped many a general dentist out when
they come up against a tough unexpected problem while treating a
patient with braces or aligners.
We have seen plenty of people happy with the treatment they have
received from general dentists, too.
I would recommend seeking 2nd and 3rd opinions if you are uncertain of
the best choice for  you.  

Mona