seen as i'm at the top of RichE's list of people to ask (nb, prolly because i spend the most time on tsr and definitely not my math prowess) i figure i best say something.
dieeiervonsaton is right - the pure aspect is very interesting but i only really got into it during the last few months (ie, when i was revising) but up until that point i'd be lying if i said i'd given it a chance and worked really hard at it. so if you work at it it does reward you, and how much you work at it will determine how much you enjoy it. but beware, the pure aspect is completely different to anything you've ever done at a-level, and the pace at which your taught it is pretty hectic. to many i think its daunting in the first few weeks, but its definitely not impossible - and once it clicks your away.
the applied side is more my thing - for a long time i was also contemplating doing an engineering degree. if you've done further maths, especially pure and mechanics modules then you'll have a good basis for applied maths at uni, and i found it easier to adapt to applied than pure purely because of said experience. however, after the first few weeks you'll be on completely new material so don't relax!
after i finished my first term i kinda took my foot off the boil. if there is anything i'd say to future students it would be do not take your foot off at all. keep work ticking over and keep on top of it. after the second term finished i had to work solidly until exams, and i did - my friends will tell you the library was almost my second home in the last month but my mark, although better than i expected left my tutors and myself disappointed. i know i could have done better and myself justice if i had worked that second term. if you want to do fulfill your potential you need to work. the workload is nothing fantastic - if you go to the lectures and do the problem sheets and revise thoroughly you'll get a decent second. however, just spending an extra hour or so during the day will make no end of improvement.
forgive my ramble, but my first year maths degree has been an experience i would not like to have missed out on, and if you enjoy it and are willing to work at it, a maths degree at oxford is one of the most rewarding things - not jut your job prospects but you as a person as well - you could do.