I have used E 'once', and am an opiate addict. Thus, I use opiates every single day (and pretty much 'all day long'). That being said - when I used E, I was under the influence of a high amount of opiates (oxycodone, to be specific). I can't say if E is better w/o opiates (however, if I had to guess: I'd say it would be more 'purely 'enjoyed, or 'the effects would be more pronounced'... if (E) is taken alone).
I did not experience any notable negative effects due to the combination. However, keep in mind my tolerance to opiates, and that I took ONE 'E' pill (and that types vary in ingredients as well as strength/s of ingredients). You didn't mention some (what I'd categorize as : ) 'key details' that would be helpful in answering your question.
1) Do you have experience with E? *anything stand out in your intake history?
2) Do you have experience with opiates? A tolerance? etc...
3) Do you have any health problems?
4) Are you asking a general question or do you have a specific dose/amount of (both opiate and E) substance/s in mind?
5) Are you referring to a specific type of opiate or opiate/s in general? * apply this question to 'E'