I have tried all those opiates due to disc problems in my back. Tramadol is unique, as it is a synthetic opiate and partial opiate agonist of the Mu Opioid receptor. While other pure opiates affect all receptors on your body, Tramadol only acts on some of them. It is a novel chemical.
Couple other notes on Tramadol: it is usually prescribed, especially in the US, as a first line pain med because it isn't scheduled, addiction profile is lower than other opiates (but addiction is possible!), and is typically well tolerated in user groups. Tramodol, as you mentioned, is also a mild SSRI, inhibiting Seratonin reuptake.
I have taken Tramadol off and on for years, it works well for me, but here is the interesting thing. For most pain management patients, Tramadol has a short usage life. It doesn't help them for long and not enough for sever pain. I have often wondered why it works so well for me and did some research a while back.
Two "side effects" of Tramadol: Vasodilation and it enlarges the prostate. Both bad things in most men. However, any vasodilation med (caffeine, nicotine, some nose sprays, decongestants) improve my symptoms. Because it clears Histimine from the body. And I suspect the enlarged prostate may mediate some of my Pois related symptoms. Semen production is different when I am on Tramadol, urinination is different. It's an interesting chemical.
I feel like I need to discuss the downsides: can cause addiction, not everyone tolerates it well, can cause jitteriness, you can't take it with other SSRIs or it can cause Serotonin syndrome and most importantly, it has a huge drug interaction profile.
http://www.webmd.com/drugs/mono-5239-TRAMADOL+-+ORAL.aspx?drugid=4398&drugname=Tramadol+Oral&source=0Please understand, I am not recommending its usage. Just want to share my experience and research.