Don’t use the included spacer if you are putting this on a Rebel 300. The brackets will be too far away to screw in or stay connected. Not sure about Rebel 500, may be same issue.
Without the spacer, the seat goes on just fine. You may need to flip the front bracket if you didn’t put it tilted side up.
The seat when you first sit on it feels no better than the stock seat, but once you ride on it, you feel the difference. This seat is more comfortable than the stock seat. I’m not certain if it’s the padding or simply because you sit a little bit differently in the saddle. There is more room to sit back a bit. Stock seat I was hurting around 50 miles. This seat I was still riding at 100 miles and not sore.
I put a Purple cushion (purple mattress stuff) on top for more comfort for longer rides. I went 250 miles comfortably yesterday. That’s my longest with this seat so far.
Cons: the underneath is all plastic, if you pull on the seat or slide off one side, you can pull the little threaded anchors out that hold the screws in place. The anchors can be pushed back into the hole but it’s not easy. Don’t pull on the seat to make sure it’s secure. Stand on the pegs to get off the bike instead of sliding off one side. This will keep the seat from breaking free. Not the most sturdy because of attachment but perhaps they will change that in the future.
Overall, I’m happy with this seat.