Rapid games are fine for improvement; it doesn't matter much what the time control is - so long as you have adequate time for "extra" thinking if you need it. Solely for chess improvement, Rapid is probably about the shortest most are willing to go: again it all varies from person to person though.
What I would however recommend is to not only play solely one time control. 25 + 10 is a decent time control for improving, but you do not want to condition your mind to only this time control, or you will have trouble adjusting to longer games (like OTB classical) or shorter games (like blitz).
If 25 + 10 is a format you are comfortable with, then by all means play this time control as your "go-to" time control. I would however mix in some longer and shorter games to though. It will make adapting to other time controls easier in the future.
That is, if you want to really improve at the game, yet you don't play any longer or shorter time controls?
I understand it's a gray area, but even if we don't go along what other sites consider as rapid time controls, then let's at least consider the tournament-standard 25+10 only. Blitz is already too fast for me, but I don't want to play long games either, as it'd too much of a time investment for me to play over two games daily, and I can't count on finding adequate opponents online if I don't play enough games for variety.