I've become a mixed bag when it comes to the filioque (although I couldn't accept it in the creed).
The first thing that I take into account is that the Creed was formulated in Greek, with Greek definitions of words.
The Creed came to us in two phases. The first phase, from the Council of Nicea, was a little more simple. It was Trinitarian and emphasized the Divinity of Christ (after all, it was in response to the Arians). For the Holy Spirit, this version of the Creed merely said, "and in the Holy Spirit".
At the council of Constantinople, the Fathers added to the Creed. In this "updated" version of the Creed, the Divinity of the Holy Spirit was emphasized to counter those who would say that the Holy Spirit was some kind of "impersonal force", or was created by God, or something. So they clarified that the Holy Spirit derives His eternal origin from the Father, and is therefore Divine, just like the Father and the Son: "Who proceeds from the Father; Who with the Father and the Son is worshiped and glorified". The purpose of this section of the Creed is to explain the the Holy Spirit is Divine and is worshiped as God.
The word used in this section of the Creed for "proceed", as some others have pointed out, carries with it the meaning of coming forth from a point of absolute origin. In other words, the Greek tells us that the Holy Spirit has His "immediate" origin in the Father.
When translated into Latin, that section of the Creed is less specific. The Latin word used is closer to our word "proceed", and is more of a general movement, not necessarily from a point of origin (although it does not preclude that understanding).
Given that there is a difference in Greek and Latin definitions of the word "proceed", and that the Latin word is less specific, there is room for differing language in Latin that isn't necessarily "incorrect". It is not incorrect, given the Latin understanding of the word, to say that the Holy Spirit "proceeds" from both the Father and the Son. It's merely saying that the Holy Spirit "has movement" from the Father and the Son. That is technically true, at least according to the Patristic citations that have been presented in this thread.
A problem arises if one were to translate that back into the Greek. It may be OK to say that the Holy Spirit "progresses" from both the Father and the Son in Latin, but to say that the Holy Spirit's "original" movement is from the Father AND the Son in Greek is not correct.
Also (and I place much less weight on this point, but I think it's worthy to at least be brought up, even if it doesn't make any difference), the filioque was added to the Creed to defend the Divinity of Christ. But it's added in a section that is devoted to defending the Divinity of the Holy Spirit. If one were to further defend the Divinity of the Son, it would be more appropriate to make that addition in the section that is devoted to the Son's Divinity.
My line of thought is as follows:
1) The language used in the Latin version of the Creed is "technically" correct in what it says about the Holy Spirit's "procession".
2) The language used in the Greek version of the Creed cannot support that addition.
3) The Creed is the Creed for the WHOLE Church, and thus should reflect the mind of the whole Church.
4) Since the filioque cannot be translated back to the Greek, and thus cannot be proclaimed by the WHOLE Church, the filioque clause lacks catholicity and should not be part of the Creed.
It is for that reason that I cannot accept the filioque. I don't think that it's incorrect when spoken in Latin, but it is not an addition that can rightly be used in every version of the Creed, and thus shouldn't be used at all. If we profess belief in "One...Catholic Church", then we must profess a Creed that is truly catholic.