Experiences with Gradescope?
I have taught several large classes (100-400 students) with long-form hand-written exams, all of which we graded using Gradescope.  I cannot recommend Gradescope highly enough.


  Is it worth having a teaching assistant help me scan the quizzes?


If you have a proper high-speed, high-capacity scanner, the actual scanning is tedious but relatively painless.  Scanning 300 14-page (7 double-sided sheets) final exams---a bit more than 4000 pages total---took my TAs about four hours, plus another 15 minutes to associate scanned exams with students.


  Is there anyway to structure my quiz to streamline the scanning process?



Have students write answers on the exam handout itself.  Scanning printer paper is infinitely easier than scanning notebook paper or blue books.
If possible, print the exam handouts with a visible border 1/4" from the edge of the paper, and warn students that anything written outside this margin won't be scanned, and therefore won't be graded.  You may also want to indicate a region near the staple; the easiest way to remove staples is to cut off a corner of several exams at once using a paper cutter.
Print and scan everything double-sided.
Gradescope automatically splits large PDF scan into separate exams if every exam has exactly the same number of pages.  For long-form exams, include a small number of pages of scratch/overflow paper in the exam handout, and scan them all, even if they're blank.  
Have a clearly delineated box on the first page of the exam where students should print their names.  This will allow Gradescope to automatically associate most of the exams with the correct students using OCR.  This also helps Gradescope streamline the manual matching of the few exams it can't match automatically.
Ask students to write their answers to each question in a designated box in the exam handout.  Then if you can tell Gradescope where each question's box is, it will display only that box by default when you are grading that question.  (The entire exam scan is still available, however.)
In particular, for true-false, multiple-choice, or short-answer questions, Gradescope can cluster similar answers together, again using OCR, so that (to first approximation) you only have to grade each distinct answer once.  (I believe this feature requires a more expensive license.)
Include a fake student in your roster, and use it to upload reference solutions in advance, so that you can set up rubrics in advance.  (Gradescope doesn't let you establish rubrics for an assignment until it has at least one submission for that assignment.)