Supreme Commander veterans will be most at home here, but a clone it is not. By borrowing bits and pieces from various other RTS games out there, this game has become it's own beast. Don't go into this expecting it to be exactly like another Supcom/TA.
Ashes of the Singularity is a mixed bag. It is best described as a combination between Supreme Commander and Sins of a Solar Empire. It is an RTS with a streaming economy based around forming groups of units into meta-units (called armies in game), and then fighting over spots on the map that take time to capture like a Relic RTS.
The campaign is honestly pretty meh. The difficulty curve spikes rapidly in places, only to be smooth sailing again for a few stages afterwards. Despite these difficulty spikes, veteran RTS players will probably find themselves blowing through the campaign pretty quickly. I had to surrender twice during the campaign, and both times I finished the stage no problem once I knew what was going to happen.
The story is predictable, rudimentary, and tacked on. The voice acting is pretty terrible, especially for the female. The in battle conversation is just text that is jarring and constantly yanking your view away from what you are trying to do. Not only is the in game text not voice acted (probably a good thing with the poor quality of it anyway), but there aren't even animations or portraits to make it interesting. The in battle dialogue can honestly be skipped, and it wouldn't change the experience much as most of the conversation boils down to, "Insert middle-school play complexity of dialogue to justify this battle." The player starts off as a "neophyte" that is brought in to assist the big boys, and suddenly about 3/4s of the way through you have apparently made enough of a name for yourself that you are considered "quite powerful." However, I didn't feel the crescendo here. It just felt tacked on like the rest. Suddenly I was strong and scary, I guess because I had won so many battles, but there was no real explanation for anything.
Overall, you aren't missing much if you skip it and just jump into skirmish or multiplayer.
The multiplayer is where this game shines. I was top 10 or 20 at one point during the beta, so I have a fair amount of ranked games under my belt. It's slower paced than most RTS (although, still much faster paced that Sins of a Solar Empire), which means success isn't based just as much on your apm as it is your strategy like most modern RTS games. On bigger maps, your ability to keep track of and control multiple fronts at the same time will still be relevant though, so APM isn't completely irrelevant.
The two races aren't exact copy pastas of eachother, but neither are they drastically different. Their units are different in both look and in the manner in which they attack, but in the end both sides tend to have units that fulfill the same role. For example, PHC has artillery cruisers and dreadnoughts, while Substrate has drone cruisers and dreadnoughts. Their method of attack is different, but they both basically fulfill the same function. The same can be said for other units. Both have a close range, tanky, t1 unit that costs only metal, and then both have a t1 longer ranged fragile unit designed to sit behind those tanks. One shoots a laser, the other shoots a rocket, but they are essentially the same thing.
If you prefer to spend your RTS time in multiplayer or skirmish, than this game is an interesting take on the genre. While Ashes isn't doing anything that hasn't been seen before, it is taking the good parts from various other RTS games and mixing them together into a concoction that is new.