While I had a great time in my playthrough, in some ways the lack of stategical importance in playing really hindered my enjoyment on things. After going through the first few levels exclusively with Jean/Aimy, I found that though I was progressing through well enough, things dragged a little despite the unique time-travel feature. Because gunning down everything in sight works quite well for basically every encounter in the game, there's never really any reason to ever experiment with anything else.
Though the skillsets of Jean and Aimy are probably the most effective at getting things done, once I started branching out into the other characters things got a lot more enjoyable. Some characters like Merlin I never really found much of a use for, but they all have their own interesting niche if they're experimented with. Zackasaurus and Lou Don Jim are hard to find uses for due to enemy overload and melee being dangerous, but they find a nice place in boss fights. Using Shieldy feels very awkward if you're trying to kill stuff, but he's great at deflecting bullets and attacks that would otherwise kill past selves. Shieldy taking Cleo's power, for example, doesn't exactly make him a powerhouse, but it does provide him with the ability to permanently stay in shield mode while adding some surrounding boomerangs for additional melee damage. It's not exactly the most useful revelation, but finding out combinations like these is quite enjoyable!
It would be interesting to see how a ranking system would be implemented, as I do agree about there being a lack of clear reasons to return. I have yet to try it, but I think Super Hardcore Mode is where the main replayability lies for those that give it a fair shot. After zooming through the game throwing countless lives away haphazardly, I like the idea of having to be more conscious about dying and having to use other characters if your mainstays die. Shooting well and surviving doesn't exactly feel bad, but it's not really where the fun comes in compared to games like Contra where that's their whole point. Trying to utilize the various strengths of the time force members simultaneously while also learning how to effectively manage time though... it provides for a way of thinking that I simply haven't seen in other games.
Considering Capy made Might & Magic: Clash of Heroes as well though, I suppose I shouldn't be surprised that I found Super Time Force's unique gameplay to be very enjoyable. This sort of came out of left field for me to be honest, as I don't pay attention to PSN/XBL games and there wasn't much word of mouth for it from what I could tell, but maybe all that's needed is more exposure. Something that a future Steam release would hopefully promise, eh?