You don't normally use Present Perfect with stative verbs (have, be, like, seem, prefer, understand, doubt, know, etc.) Here's a longer list - in general, they apply to states that last for some time.
In some contexts, such as "How long have you had/been having these symptoms?", there's no real difference. Arguably, been having calls more attention to the fact that you're still having the symptoms, but I doubt many people would consciously either make or hear that distinction.
A "rule of thumb" for to have is: when it means to experience, you might want to use Present Perfect; when it means to own, you almost certainly don't.
Here's an example for to be using the "slightly unusual" Present Perfect in a construction which is perfectly valid, and is probably the most succinct way of expressing the intended meaning...
By now the new cook will have been being introduced to her duties for several weeks.
Yes, the above sentence sounds okay. Or otherwise what sounds okay to me is:
We are pleased to share, our knowledge in network device management,
with you. Please, find in the following, our experience in the RFCXXX
implementation.
Best Answer
But always contradicts, and is therefore almost always negative. You can use it as "positive connecting word", but only if it's contradicting a previous negatively-oriented statement:
If it's not contradicting anything, don't use but. Use and.