I love cats and have always owned one until a few years ago but yes, they are a pest to gardeners at times.
My experiences and tips:
1) Ultrasonic device - works well on my parents balcony, but if you have sensitive ears (like me!) you can actually hear it. Also uses up a lot of battery unless wired into mains, as you'll activate it every time you are pottering about too.
2) Silent Roar never worked for me and smelt foul, as did everything it touched!
3) Pepper Dust and Organic Cat repellent from Growing Success (based on garlic powder) both VERY effective especially for sprinkling around newly seeded beds and pots (mine were marauded by cats until I found these). Pepper dust downside- disappears at the first rain. Garlic powder - last longer but you can see the whiteness of the tiny spots against the soil (if that bothers you).
4) I take lengths of bamboo cane and chop them to 30cm lengths, then stick these in the bare ground all around my garden - they can't get past them.
5) For larger plot of seed bed, I cover with a big, fine meshed cloche (folds up and goes into shed easily for rest of the year). It fixes into ground with tent pegs and works very well.
I also shoo them away whenever I catch them at it and throw water in their direction - keeps them away a bit, but one is very brazen and just waits 'til I'm out
Lov'em dearly and would never harm one, but they've cost me a fortune in ruined plants and seeds - until I wisened up to all the above!
Good luck - remember they are creatures of habit, so whatever you do/use, you have to be religius and maintain your habit, to deter theirs.