Do 180g vinyls sound better or is it a myth ?

Whatever weight vinyl you choose to listen to, I believe it is essential to clean it before ever setting it on your platter.  Even a new record needs to be cleaned with a good record washer.  After that, good storage is important as well, anti static liners are a must.  If the jacket is important to you, as I believe it should be, a plastic jacket cover helps protect it.  The difference between a clean record and a dirty one can be the difference between night and day.  

My take:  a 180G pressing means that someone at some point at least thought about representing it as higher quality, meaning there is a greater probability that care was taken with the record production and pressing. But that is all it is - you can still get a very crappy record as well.  I have ordered and returned 3 different copies of Hunky Dory (Bowie) on Simply Vinyl before I decided the whole run must just not be good.

I have heard, but have no way to test, that 180G vinyl is less likely to warp, which in the long run would be a kind of quality issue.  I have also read that you can play 180G vinyl more than once in a day without the concerns associated with lighter pressings, which is likely a related point.  

I'll be able to post more opinions once I get my cartridge replaced - cleaning lady snagged it last week - hard to play vinyl with no stylus.  And I was only about 20 hours into my Ortofon 2M Bronze.  Ouch!

Ouch indeed on the little accident . I have just just ordered some vinyl cleaner so will be treating my collection to a clean when I get them. I have a few 180g pressings and quiet a few new artists are having their albums released on this. I'm on the impression that every little increase in quality in any component can help to a better experience. 
However I have some old vinyls that sound great. And to me playing a record that is older than me, but sounds close to perfect is really satisfying . The more I understand about how each component works, the more I enjoy the experience 

There are many cd based systems that sound better than most vinyl based systems ...

Thats a substitution of meanings. Yes, if you compare week analog vs. good DAC then yes it is possible, however in equal systems LP will always outperform CD It will have more refine, organic, more natural sound, one more step towards real sound, instruments will have more natural tones and will sound more real, more air around instruments, hi's more natural etc.,  Though I haven't heard in my sys. $15.000 and up DAC's, i.e. MBS, Total DAC, new Berkley etc., nor do i care cause it doesn't make any sense at all price wise anyways. If you take exactly the same recoding one is on LP and one is on CD, LP will always sound better (BTW, in this specific case not different but better), even if that late re-re-issues, possibly from HDD, still, it'll sound better than CD. 180g LP is total waste, I think they try to substitute quality, perfectly flat vinyl from 50-60's which as i understand is no longer available with more massive one. BTW, we all should start sampling recording of our sound in support of our claims ;~). It also depends on one's listening experience, I personally changed my mind on that issue once for the last 15 years and BTW I started listening to vinyls only in 2001 i think and converted completely only in 2012 i guess. Still have 3 DAC's and about 4 transports/players, no SACD though. My CD's plays like never before, but  but now when i'm listening to CD i know exactly what i'm missing in terms of sound compared to same album on LP - mostly refinement. Setup, especially room acoustic is crucial for both formats of course. I'm a fast learner, so it took me just 25 years to understand what is going on.))))))