Is researchgate still useful? It seems that researchgate is gaining some popularity in recent years. However, there is some criticism on their habit of sending requests and unsolicited spam emails?. What is your experience of using researchgate? Does it help promote your research and get some readership?
Depends on your purpose.

If your aim is to track the publications of a given academic then I would recommend these in the following order of preference. Some of these may surprise you:


Google Scholar;


Google Scholar seems to capture more publication outputs any other source. This is partly due to the scope they have for including different publication types, such as  reports and discussion papers that many other platforms exclude. They also have greater capacity to show citations to those publications. This will often lead to a higher H Index on Google Scholar than on other platforms;


Microsoft Academic (Microsoft's weird equivalent to Google Scholar);


Microsoft Academic is the almost unheard-of equivalent to Google Scholar. It has a different look and feel but does more or less the same thing, but less well. It does not seem to capture as many publications as Google Scholar and seems to require more intervention on the part of the academic to keep their record up to date, and is therefore less reliable;


Twitter;


Whaaat? Twitter? Yes. Twitter. Many younger academics send a tweet whenever a new publication of theirs becomes available. It's as close to a real-time notification of new publications as you can get with those that do this consistently. Not very good for reviewing previous publications but great if you are following the latest from a particular academic; 


Academia.edu; 


Academia.edu is hanging in there as a platform despite being behind a paywall for many of their features. Many academics still maintain a record of their publications on this platform because it offers reasonably good analytics. But they have to keep it up to date themselves, which can often mean an inconsistent record of publications and often a significant time lag in publications being added;  


ResearchGate; 


As with Academia.edu above, only their analytics aren't as good, so there is less incentive for academics to keep this up to date.


Linked In;


Many academics use Linked In for their professional networks. They typically want people in their network to see what they are up to so they often keep a selective record of their publications there as well;


Their own departmental web page;


Updating their own department's web page is normally the last thing on the list for most academics. It's often a pain for them to update because they typically don't have access to this record themselves and are therefore reliant on the lazy-ass staff at their own institution to (reluctantly) update it for them - or worse - rely on the deeply flawed system their institution uses to track and report their research outputs. These often deliberately exclude publications which do not count for 'points' in whatever research incentive scheme they use. Don't get me started!  

Finally, there are a host of platforms claiming to offer a definitive list of publications attributed to each author (including Orcid, Publons, etc). Orcid really exists as a means of attributing a unique identifier to each author so that publications might more reliably be attributed to the correct author. These have a role to play but they are not really designed as a means for making an author's publication catalogue accessible (and these platforms are simply awful to use, so many academics don't bother with them at all once they have their author ID number). 

The main differences between these pertain to how much information is included and how they are updated.

Platforms like Google Scholar are more reliable as they do not rely on the academics themselves to keep their records up to date. Academia.edu and ResearchGate can still be useful, particularly where academics make the full text of their publications available through those platforms. Twitter and the others are useful for keeping up with the latest and getting a sense of what they think is important.

Try them out and see :-)