I was on methadone for 2.5 years, have been on buprenorphine for over 4 years now, took antidepressants as well but for much shorter periods of time and in no way methadone or buprenorphine feels like any SNRI or SSRI antidepressant, and definitely buprenorphine is much different from methadone as well.
As for methadone, I can basically repeat what siq described, although all the side effects obviously kick in with time, at the beginning methadone was a very laid-back sedative opioid with no rush for me, I started taking it to slowly taper off heroin, but ended up taking it for over 2 years, and I honestly didn't register the point in time when it actually started to make me feel like a zombie rather than provided a steady opioid effect in the background that helped me to go throughout a day without anxiety (although it wasn't really in the background as the effects of methadone were easily observable to people around me, constricted pupils, "dead" look in the eyes, slow thinking that only got worse and worse with time etc.). So at first methadone did make me feel better and was an OK opioid for my needs back then - that is get rid of anxiety which played a large part in my depression, but in the end it was devastating. My daily dose was 40-50mg/day most of the time (I weighed ~50-55 kg at that time being 170 cm tall), but I also was on 6mg of clonazepam a day, so both drugs played a role in how I felt.
Buprenorphine on the other hand is completely different. For one year I took buprenorphine (8mg a day) along with clonazepam (4mg a day taper down to 0.5mg before switching to 15mg of clorazepate and quitting), and if I were to sum up the side effects of methadone and clonazepam I must say judging by this period methadone was largely resposible for the emotionless "zombie" mode I was in as just as I started taking buprenorphine, I immediately started feeling better, my mood got much much better, I was almost euphoric for the first few weeks of bupe maintenance (but not euphoric like after taking opioids or stimulants, but simply happy with life); clonazepam on the other hand much more impacted my memory which started getting better quite rapidly after I quit benzodiazepine before I even got relief from the extreme anxiety due to withdrawal. Anyway, getting back to buprenorphine - with time it becomes a drug that works mostly in the background, which often may also lead to a trap where one tries to get more relief with it by taking more and it doesn't work that way, if you stay at lower doses, it seems to work more like a regular opioid but in my experience feeling more opioid effects from bupe always leads to wanting stronger opioid effect which again cannot be achieved with bupe, so eventually I got more or less stable at 6mg. If you manage to sort things out in your life, bupe is a drug that allows you to live without withdrawals and cravings, and, other than for the time when you have to take it and hold under your tongue for 15-20 mins, without really noticing you're on it. It definitely does have an impact on you in a way similarly to other opioids, e.g. lower testosterone, constipation, constricted pupils (yet my eyes don't look as emotionless on buprenorphine as they looked on full agonist opioids and especially methadone!), maybe can even add to your depression and anxiety if it was pre-existing, but like I wrote earlier, if you sorted out your life after the hell of opioid addiction, it is more helping. I'm far from thinking I could spend the rest of my life on buprenorphine, but after 4 yours it doesn't seem I will quit it this summer either (for many reasons, I'm not really fully stable, I'm depressed, and I lack spare time to go through the withdrawal), so if I were to take some opioid for the rest of my life, I'd choose buprenorphine over methadone any day. Thanks to buprenorphine I managed to quit clonazepam, I managed to continue studying and get a job at the uni, which all in all finally let me live an independent life to an extent, as I look at it - it's not really that much, far from even a minimum that could make you satisfied with life but it's still much better than living day to day coping drugs.