Not personal experience but experienced eye/opinion -
This is an old fashioned style of seat. It doesn't offer much at all in the way of side impact protection compared with other models on the market today, the support is especially lacking IMO for children in the harness stage. It doesn't recline for naps etc.
It can be easy to make a mistake when fitting the seat in the car, and the harness is not very adjustable and has a basic way to adjust it (uninstall seat, rethread harness to new slot). In general, the mechanisms to adjust the seat are poorly designed and potentially leave gaps between the different age stages (e.g. children within the overlap between two age stages may not fit well in either mode). These things are important, because if you don't fit or adjust the seat right, it won't protect your child as it's designed to. And the protection of this seat when it's used perfectly is already quite basic.
Plus points are that it has the serpentine belt path rather than the one where you thread the seatbelt through behind the seat only - this is more secure, though only if it's done correctly. And it's very light.
But honestly, I would not say these outweigh the downsides - it's a very basic seat, essentially just a backless booster, with the only function of the backrest being to hold the 5 point harness in place. It would not pass the latest safety regulation on several counts, and Cosatto's supplier have changed the seat significantly for the updated new regulation version.
If you are looking to buy a seat and £75 is your budget, these are similar type, and again not the best on the market, but better bets for this budget, IMO:
Kinderkraft Comfort Up i-size
Joie Elevate R129
-- Both are 123 type seats similar to this, and are fairly basic in design but have more up to date safety features and the newer safety standard.
Kinderkraft Safety-Fix 2 - similar to the others with the added benefit of isofix + top tether fitting.
The current cheapest offers on Group 1 seats or high back boosters that I would recommend start at about £90 - this may be because Black Friday is coming up soonish. The cheapest 123 seat that I would recommend is about £150 (that is already an offer price).
If you already have it or have been offered it by a friend and you are not able or don't want to replace it, to make it as safe as possible, first of all if your child is in the 5-point harness age, check their shoulder height against the harness and choose the slot closest to their shoulders. If they are between two slots, choose the higher one. If they are taller than the top slot, the seat might not protect them adequately.
I would fit it in the middle seat if it's wide enough and there is a 3-point belt there, and make sure you watch fitting videos to see the method of fitting. The Smyths one is the clearest. Make sure you kneel into the seat to get it tight as well, and then tighten the straps on the child until they're really snug to their body. Don't put them into the car with coats etc on.
In high back booster mode, make sure both parts of the belt go under the armrest on the buckle side. On the other side, there is a little plastic belt guide which is supposed to sit in the top harness slot - this might make the shoulder belt too low in some cases. The seat is only actually approved up to 25kg in high back mode and it is likely to stop fitting well in this mode at somewhere between 5-7 years. At this point the seat can only be used as a backless booster - and then, only if the child weighs more than 22kg. Again, if your child sits between these two points you may have an issue.