Tora has been taking Sentinel since she was old enough, and she's now 12 years old. Never had any side effects, and she eats it like a treat. She did get a case of worms (sorry, don't remember which) when she was young, but Sentinel covered the vet expenses, and she recovered quickly.
Max has been on Sentinel since we adopted him last August, he shows no ill effects from it and also eats it like a treat. He's 4 years old.
We use Sentinel...no flea problems and no HW problems. Everyonce in a while Anna may get a tick (only her, not Dunc) but it's weak and easy to remove. I've began supplementing with Bug Off garlic to help with ticks.