Would you ever attend a...?

[DavidB.]

The "for worship" part in the question restricts my response to Haredi Orthodox and Modern Orthodox synagogues - a difference most of us Europeans don't have the luxury to make to begin with - and to Sunni Mosques, which do not pose any halachic problems regarding prayer either. I would probably attend a Muslim service in a mosque before attending a Reform service (never attended either though). I am not quite sure about the halachic status of non-Sunni Mosques.

I would - and do - visit other religious places out of interest when on holidays, though. Frequently visit churches when abroad and have occasionally attended parts of services too, not to engage in worship but to observe how other people worship and what their tradition looks like. (This is not allowed in Judaism though.) In addition, I attended the funeral ceremony of my non-Jewish grandfather in his Catholic church - not sure if that counts as worship.