I have a Safety First one and a Mothercare's own one , essentially the same seat but the latter has a 5 point harness, the former a 3 point harness. Got them both from e-bay and use both daily. They are also great to fold up and sling in the car for visiting restaurants/other people's houses and also for daily use for making a toddler feel they are sat 'at the table' rather than in a high chair.
I have a TOMY one. (sorry no link, crashed my computer trying to do one...). I have used the others, and like the TOMY the best because you can put the tray on and off with one hand (the other one being used to restrain your sqiggling child). It also comes apart enough that you can fit it all in the dish washer. It's great having a booster chair vs a high chair. We used to take it with us to restaurants as well as it's relatively portable (folds into inself to form a small brief case like thing. We've since got a cloth blow up one for restaurants, but it's not sturdy enough for every day use. Please note that for the straps to hold properly for all the booster seats you need to have chair backs and seats that are relatively rectangular and not wedge shaped or rounded.
I think if its one with straps you can use it instead of a highchair. My sis used one from about 6m as it was more convenient than the high chair.
Ours is a bit like a block with side, no straps so we're only using it now (2 years)
with dd I only used booster seat, never had a highchair. However ds is a different matter and he nearly toppled the whole dining room chair over with his vigorous sitting style!! Had to splash out on highschair for him .
dd used hers from 5 months as did ds. she used it till about 2 and half, he used it for 6 weeks....
I did have one - called a handbag highchair. Its OK when they are small but once your child gets big or very wriggly its just not very secure. Its a large bit of cloth that basically tieds around the back of the chair. The cloth comes down the chair back and over the seat. you put your child on it, pull it up between their legs and tie the ends around the back of the chair.
We used it on holiday for 2 weeks when DS was 6 months but as he was so large he grew out of it. I got it in a very large Boots