We have purchased travel insurance from multiple companies in the past. We never had a problem with any of them until we bought Allianz.
They would not honor the pre-existing medical conditions clause and denied our claim based on a loophole.
I had purchased the policy online through insuremytrip.com (a great company by the way) -- discussing the case with them today, they're forwarding it to a claims advocate.
But I wanted to warn people against buying Allianz. I found out that even if they do approve your claim, they can 'pay' by giving you credit towards future insurance policies through Allianz instead of refunding your money!
I've learned the hard way -- read ALL the fine print, even stuff you didn't know was there. Keep scrolling / reading to the very end of the policy, especially the 'General Exclusions' which they hold against you.