I have the older TZ3 which has a 10x optical zoom. It's pretty good for a P&S. Here's a sample taken at max zoom, aprox. 270mm in 35mm equivalent. I was pretty far back when I snapped this. You can see the zoom range is pretty good but sharpness isn't so hot. Mickey's a little blurry. Or perhaps my focus point was off. I was holding the camera above my head to get over the heads of those in front of me.
The thing to keep in mind is that the two cameras you're comparing are really different types. The FZ28 is a bridge style, the TZ5 a compact point and shoot, so there are some other differences between the two other than just max zoom range. I think overall, based on reviews and comparisons I've seen online, the FZ28 is a better camera than the TZ5. The difference in lens sharpness between the two was quite noticeable, the FZ28 being the sharper of the two.