That's a difficult question to awnser... I'd ignore everyone who parrots on about the 'relaxed and friendly' atmosphere etc. etc. etc... That's not actually Catz's reputation, that's what they say about *themselves* in their advertising, which is purely down to the fact that it's a modern college physically.
The JCR IS a friendly and relaxed place in terms of its members, but then I've yet to come across any in Oxford that isn't.
In terms of the college itself, I wouldn't describe it as the most student-friendly college... Facilities are generally rather poor. In order to pay for anything (including washing machines and even vending machines for some insane reason) you have to use a prepaid 'girovend' card which you upload money onto. (You can pay in cash in some places such as the bar and in hall, but it's more expensive.) This wouldn't be too much of a problem if it weren't for the fact that you can only upload £50 at a time; if you're short of cash and just need £2 to do your washing, or even 30p to buy a Mars bar, it's £50 or nothing, which really can cause problems. Additionally, there are very specific times during the day (and not at all on Sundays) when you can actually put money on your card even if you do have enough; the cashier is frequently away from her desk and the queue is often so long some people have to come back the next day.
We are very lucky to have a college gym onsite, especially one that's open 24 hours, so you can work out at night if you want. It's not exactly high tech; the machines are a little rusty, but it does the job and is more than most colleges have. (Though not all)
One thing about Catz that is difficult to really understand until you've been there (and which is definitely not friendly or relaxing in terms of atmosphere) is that there is very little indoor space; apart from the main JCR (and the offices and SCR which don't really feature much for undergraduates) all staircases open out directly onto the quads outside. The JCR itself has a tiled and often sticky floor, and is very large, which makes it feel rather cold at times. There isn't really any decent cafe area where you can get a coffee; the 'buttery' is only open for two ridiculously short times a day, and would probably be put to shame by the average tea room in an oil rig... There are no comfy sofas, just a large vault like room, empty but for a big tv and lots of institution-like chairs. The place warms up quite a bit during episodes of 'neighbours' and 'the simpsons' etc, when there are more people in there, but otherwise it's a rather hostile room to sit in. It's not the sort of place you'd feel comfortable reading a book, for instance.
The mobile phone reception in college is very poor; around half of student rooms are without any reception at all.
Finally, with some exceptions, a lot of the non academic staff are very rude to students; you will be frequently reminded that as an undergrad you are at the very, very bottom of the pile. In short, be prepared for bespectacled receptionists to look down their noses at you if you go into the office for anything, or to be shouted regularly at for walking down a recently cleaned floor, getting in people's way, asking the lodge for favours, looking shifty, breathing, etc.
Anyway, the above is why I (personally) feel that Catz, although not without its charms, is not a particularly friendly or 'relaxing' college, and certainly doesn't deserve to be singled out for being either. If anyone can give *specific* examples of why the place IS friendly or relaxing, please do post them; I'm not out to badmouth the college, but I do feel that prospective students be informed of the reality of college life (good and bad) rather than the idealistic propoganda the college spews out, and for some reason people seem to assimilate without really thinking about it.