I taught Applied Math 1, Sequential Course I and Sequential Course II under this semester block system.  One big difference between what it sounds like you have and what I had is I had 90 minute blocks PLUS I met with each class for an additional 45 minutes 2 times in a 6 day cycle.  It was really a great system in that there was plenty of time to do in depth investigations of topics as well as reteaching when necessary.  The students did really really well on the exams.  The last year I did it I had an acclerated Course II group in the fall who took the exam in January, and then I had them the following fall on a regular full year schedule in Course III.  They did not suffer from not having math spring semester freshman year.  In fact, 3 of the 4 students in my class who earned 100 on course III that year came from the semester block the year before. 
Of course, those were the acclerated students.  The non accelerated students still did well in their course, but I don't know how they did the following year b/c I didn't have them again.
It takes a ton of planning to do a double period.  You can't just lecture for 2 periods straight...
I'm not sure I would have liked it as much if I only had 75 minutes a day.  We also were 'teamed'.  There were four of us on the team (math, English, Global 9, and Biology) and we met once a day.  I had half the team in the fall and half in the spring.  The students always had math and English one semester, Global and Bio the other semester.  There were plenty of advantages to teaming as well.
Another advantage was only having 45-50 students at a time.  Much more time to focus on each student, call parents, etc.  I'm not sure what schools could afford to run such a system anymore:(
Elizabeth Waite
AMTNYS Vice-President

-----Original Message-----
From: Iva Jean Tennant <tennantij@aol.com>
To: nyshsmath <nyshsmath@mathforum.org>
Sent: Thu, Oct 25, 2012 7:56 pm
Subject: Re: Dear Teacher, Johnny Isn't Sick. He's Just Boycotting the Test. - NYTimes.com

Binghamton City Schools did this for several years. Liz Waite actually taught courses under this schedule. She would be a good one to ask. They have stopped doing this several years ago. I do know that one problem occured when a student had algebra 1 first semester freshman year and did not have geometry until second semester sophomore year--way too much time between math courses.
It has lots of advantages and disadvantages. 

Iva Jean Tennant
President of Association of Mathematics Teachers of NYS
Past President of AMTST
People to People Student Ambassador Delegation Leader

-----Original Message-----
From: William Wickes <wickes@optonline.net>
To: nyshsmath <nyshsmath@mathforum.org>
Sent: Thu, Oct 25, 2012 4:12 pm
Subject: RE: Dear Teacher, Johnny Isn't Sick. He's Just Boycotting the Test. - NYTimes.com

Could you or anyone tell me if you have ever heard of offering Int algebra, geometry or alg 2 trig, in a 75 minute block schedule for half of the year….opinions are welcome…we are doing it and it is frightening to think that I must cover all of the curriculum in half the year….thx   BW
From: owner-nyshsmath@mathforum.org [mailto:owner-nyshsmath@mathforum.org] On Behalf Of Evan Romer
Sent: Monday, October 15, 2012 6:22 PM
To: nyshsmath@mathforum.org
Subject: Re: Dear Teacher, Johnny Isn’t Sick. He’s Just Boycotting the Test. - NYTimes.com
The thought I had on reading this was: how can they give field test questions in the fall? The students haven't been taught the whole curriculum yet, so how can the field test results be valid? A Pythagorean Theorem question may field test as a very hard question simply because the students haven't learned that topic yet.
I suspect that the answer is that they need to have Common Core questions for the April 2013 assessments, and they don't have enough questions field-tested yet, so by God we're going to field test them even though the results won't be valid.
Evan Romer 
Susquehanna Valley HS 
Conklin NY