Definitely an opinion. Try to remember Serato DJ was not just released. Many of us have been using it since day one for over a year. There are also many of us who since day one have not had ONE major issue to date. I have faced one issue total to date and this issue is something I can work around for the moment. The biggest (non support) gripes are not having DVS with their hardware of choice, lack of midi panel & lack of support with device whether it be an unsupported CDJ or a SL1, etc. ALL of these issues are avoided by purchasing the proper hardware. Some other (non support) issues are changed GUI or features from SSL which I am sorry to say is a matter of preference and only became an issue recently with the drove of SSL users switching late in the game. Some happen to like the changes brought on by SDJ and recognize the fact that it is SDJ and not SSL so having it be identical is not the direction Serato would necessarily want to take keeping the concept of moving ahead in mind.
All that being said SSL is on its way out so had plenty of years to get where they are today. Let us not forget it was Serato who got it to where it is. Keeping that in mind giving them a fraction of that time at the very least before making premature judgements would seem more than fair to me. I understand there are features that this person wants or something that this person hates BUT Serato tries to keep the community at large happy by going on a majority basis and if things are NOT the way you want them there is a chance you are not in that majority. I hope all goes your eay and you get what you want out of SDJ and if not enjoy SSL. I am just trying to shed some light from the other side and there are indeed plenty of us who could not be happier. I WILL say this once you embrace SDJ and 'level off' so to speak you will realize the endless possibilities that were not even remotely possible with SDJ. Performance pads & their respective features alone are reason enough to never look back.
In terms of TTM57SL please keep in mind even though it is only 3 yrs old to you when it comes to native Serato mixers there isn't an older mixer available not counting the MP4 which I don't think can be counted as a real DJ mixer and was discontinued years ago anyways. When SSL is officially no longer supported Jan 01 2015 the mixer will be a yr short of being a decade old. This is all FACT.
Here comes the OPINION. If you like SSL then stick with it. On that same date Jan 01 2015 SSL will work just like it did Dec. 31 2014 and every day before that so this only means so much. The fact is Serato will no longer release updates for SSL and no longer release any new products for it. If you are happy with what you have and what you can do with what you have then stick with it. At least you have the fact that any/all work you do to your library (which let's be honest is one of the most important aspects of DJing) will be applied to SDJ when you do decide to upgrade.
In response to the SL2 being the same as the TTM57SL.............. besides the many other things that make these two very different one is USB 1.1 and one is USB 2.0 which is seemingly the main reason the TTM57SL (and the SL1) were not added to the supported devices for SDJ.
The mixer certainly mad an impact on the industry though and has absolutely earned its respect.