I love my Ion
Ion's are great starter to mid range markers. Any problems that I have had have been originated by me not putting the thing back together properly. I'm not having a dig at anyone here, but it is so simple to maintain, and most problems can be corrected easily.
Here's my 101 on upping an Ion, so at least you can get a feel on what you can do to an Ion if you get one. Mine's up'd to the hilt and fires so sweet, and I have not had a problem with it for ages.
http://www.p8ntballer-forums.com/vb/showthread.php?t=81735
Their are other markers to consider, but if you want to start off by buying a stock tourney ready marker for either £130 new or less than £100 used, then the Ion is the one for you. You can then choose to modify it gradually in line with your prerefences, or save your pennies to upgrade to something more expensive in the future.
Oh, some people hate Ion's becuase of the problems they have had with them, bit for the money you can't knock them.
Use your search button to have a look at similair threads posted regarding the Ion, and don't be dismayed by the negative comments flying around. All markers have problems, but it is so easy to blame the Ion due to the low price and apparent plastic spoon type body it has.