BizTalk needs to be used correctly,
I am a BizTalk developer and my experience with BizTalk is quite good.
Its reliable, performant, scalable, contains a lot of built in architectural patterns and build in components to make integration easy and fast, you get security, retries, secondary transports, validation, transformation etc... and what ever you dont have build in with BizTalk you can easily customized with .NET code, its basically a hard earned integration system and you get all this in one box.
BUT you need to know how to implement BizTalk correctly, not once I came across solutions that where implemented and often also architected incorrectly.
but the real benefit of BizTalk is that you can implement small solutions and scale up whilst most other integration systems from big vendors will only sell a whole integration pack which can cost much more.
BizTalk is considered the most complicated server from the house of Microsoft.
So any body saying BizTalk is not good dosent know BizTalk period.