I've done a few of them in the past, and yeah they definitely suck. I find the insinuation that your ability to succeed in a certain career path is defined by your numerical and verbal reasoning under time pressure a little bit ridiculous.
Obviously you need intelligence to succeed, and I would sincerely doubt anyone who suggested that wasn't the case, but in the end, the way you apply yourself to tasks, your attitude and inspiration is always going to come out ahead of calculating compound interest once you're in there. At the end of the day, getting your foot into the door is often the hardest bit... from then on you've got a good base that you can work from.
I wouldn't worry too much about them - I know people who work their arses off trying to improve, but the tests just aren't designed that way, it's not a test of experience... and I know people who get mates round to cheat on them as well, which I don't think is much of a tactic either. In fact a girl I spoke to who cheated on one of them did abysmally, despite her and her mate both getting solid A grades at A level maths. Quite surprising actually, I can't help but think having someone there to help weighed her down to be honest.
Last job I missed out on I did something similar to you, very final minute of my very final interview and they asked me "How did you feel when you walked in this morning" (it was an assessment day with other applicants to the firm), and in truth I'd found myself amongst a group entirely made up of final year uni students, uni graduates and people already in industry (not rubbish either - Oxford Uni, LSE Uni, Microsoft I remember), compared to me, half-way through my A-levels. So of course I began my answer truthfully with "Intimidated", and realised about half a second later I really shouldn't have said that, leading to minor panic, which made my answer get even worse. No surprises when about 2 weeks later I got a "Thanks but no thanks letter". It's so unbelievably frustrating when you know that hours of research, a fantastic first interview, a fairly decent assessment day can theoretically all be chucked away in an instant .
Having said that though, after all of that fell through (similar jobs with similar firms also in central London), courtesy of a couple of extremely unlikely connections someone recommended me for a job sorting out the IT dept in a small school, and to be honest I'm really glad it all went wrong at the beginning based on how things have turned out. I'm working fairly locally, with a very friendly crowd who don't demand too much and place a lot of faith in me - and on fairly similar wages. I just couldn't have expected all of those from the places I was looking at originally. Admittedly, the experience at one of the London firms would've been invaluable, but then I've probably got more responsibility here as well so it all balances out. I know it's a "same-old tune" situation, but I'm genuinely pleased with how things turned out in the end... you never know quite what's going to happen. Just takes someone with a bit of vision to see through all the crap and give you an opportunity.