well i've read the first 3, and haven't really touched the fourth one. maybe the stories may get a little more detailed and go deeper, but it's in no way comprable to LTR. but like you said their not exactly on the same playing field anyways.
yes the books are extremley long, but they are written at a very low reading level and can be easily finished within a day. then again anything that encourages children to read is always a plus, i just would encourage reading something of more... emm... literary merit if you want. something that would broaden their knowledge of human nature or vocabulary at least. but i guess that is the whole point of commercial fiction is for entertainment purposes so i will stop harping on this now futile subject.