Thermasilk is it good?

I'm not too sure if this is the case (others can back me up or disagree) but I don't think any "two-in-one" products are very good for your hair.  If you have dry, coarse and brittle hair, you probably want a repairing or gentle shampoo with a deep conditioning conditioner.  Ones to try are Tresemme Strengthening, Panteve Pro-V w/ amino acids, Garnier Fructis Long & Strong or my personal favorite, Paves Professionals (You can buy this at Walgreens). 
Others here may suggest Conditoning Only (CO) as well.
Do you have very long hair?  Did you want to save yourself some time and this is why you wanted the two-in-one?  How often do you flat-iron??
I have a flat-iron but I only use it like once a month if that.  I have the Chi Turbo Straightener.  If you straighten a lot, I'd recommend a heat styling protector.  Matrix Sleek Look has a product specifically made for flat-ironing.  You spray it on each section of dry hair and it protects it from the heat.
I just think that a lot of the time, the two-in-one just kind of conditions while it cleans and then you dont have the extra conditioning at the end. 
I really really like the Paves Professionals line.  I read an article about it.  It's created by Jessica Simpson's Hair Stylist and it's GREAT.  It's also not that expensive (about $6 a bottle).  I'd try that!
Hope this helped!
Love,
Christine