khemix, maybe apostol is not as much fun as spivak, but it is very scholarly and any strong student will greatly benefit from it.
well actually your post does put the blame where it belongs if you read it twice, namely on your teacher and your preparation, not on apostol. unfortunately you attributed your bad experience to the book, not to your lack of readiness for it.
but that's all right. you are well served by spivak. but apostol is a great book for the right student and teacher. it was the first book used by one of my best colleagues at MIT, and he liked it fine. i also used it for returning high school teachers one summer and was very impressed by it. but it is an odd choice for eleventh graders.
as to rigor, here's a tiny example, compare the proofs of the intermediate value theorem in apostol to that in spivak. see which is clearer. to me apostol was a bit simpler and better, although both are rigorous.
one fine feature of apostol is that he denies the student the crutch of starting out using the fundamental theorem to do integral calculus, and thus insures ones learning the meaning of integration. i.e. he does integration first, as it should be done. when the derivative is done first, hundreds of years out of order and context, the average student never again makes any effort to understand what the integral means, but always uses antidifferentiation to do integrals.
The student never learns that this approach does not always work, because it is the only one he ever uses. Apostol gives a thorough study of the integral first, so the student learns that the integral really is something quite distinct from an antiderivative, and can be studied very thoroughly without derivatives. Then when the derivative is introduced and the FTC, it really is a connection between two different ideas, both of which the student has learned.
In the usual approach the student never learns what an integral is, and no amount of explanation can make it clear after the fact. Once the average student has the FTC, he will never again listen to an explanation of what an integral is. Then when he meets an integral that cannot be done by the FTC he is lost.
Indeed many students even think that a step function is not integrable because it is not continuous and so the FTC does not apply. How foolish is this? That's like saying a figure made of two rectangles does not have area!
apostol gives the proof, due to Newton, that any monotone function is integrable. this very easy proof makes the idea of integrability quite clear, and shows it is not dependent on continuity. most books omit this theorem, and rely instead on a less easy one that all continuous functions are integrable, which they then leave to the appendix, or leave out altogether.
spivak's approach is to do the hard technical theorems first, allowing him to prove that continuity does imply integrability. this is now a standard advanced approach, but i think apostol's historical one is even better pedagogically.
by the way notice that Newton, the man who invented calculus for application, also supplied the rigorous proof mentioned above. rigor is a tool for making sure the applications we make are actually correct.
I suggest you try to realize that just because you yourself had trouble learning something at one point, it may not be because the book was a bad one. maybe you just tried to read it too early in your career. otherwise you run the risk of closing yourself off from some great sources that will be accessible to you later.