Magnet has a ton of different uses. First of all I'll direct you to the "How are you using magnet?" thread already in this subforum.
Magnet thread:
http://smashboards.com/threads/how-are-you-using-magnet.334150/
Really great video showing some of it's uses:
http://www.youtube.com/watch?v=lZZYTPQ1mH0#t
I love using the magnet and IMO it is one of Ness' best tools. It can be used as a combo starter, extender, momentum shift, aerial stall, mindgame tool, and recovery tool (think I listed everything I use it for).
The way I use magnet most is for comboing. On most characters Ness can dthrow -> magnet -> fair -> regrab OR on some floaties a more reliable string is dthrow -> magnet -> weakspot nair -> regrab. Typically if I get a raw magnet in at low %s I immediately djc uair which generally leads to fair, another djc uair or dtilt -> grab. Is so amazing in his combo game. I love it.
Magnet also has a lot of nifty uses with wavebouncing or breversal, whatever you want to call it. Calabrel and SinisterB do this a lot (at one point in the video above you'll hear Daz refer to Sinister's use of it as an "aerial wavedash"). WB Magnet allows you to completely reverse your horizontal momentum while stalling your vertical momentum (I use the vertical stall in my recovery in a similar fashion to how Lucas uses his). This works great as a mixup, techchase, mindgame, etc. throwing out a magnet toward your opponent and then wb'ing the other direction can sometimes be used as a bait which you can quickly djc aerial out of to punish. If you knockdown your opponent it's great as a stall to wait and read their tech and then accordingly jump and djc aerial. I know GMaster has said something before about a mixup he does in this situation with dair. Can't remember what it was though.
I could keep going but unfortunately I have to head to work. I stream smash very often so check out my stream sometime (I'm usually always playing Ness). I love talking Ness tech. Hope this was somewhat helpful.
EDIT: Okay off work so I'll expand a bit. Another thing that is a bit controversial among Ness mains is grounded magnet. I personally like but I know others (NZA, KiddKoopa) don't care for it. At lower percents you can wavedash out of a grounded magnet (considering it hit your opponent) and on most characters it's a free grab. Similar to waveshine. Something you'll see Calabrel and I use is grounded magnet -> jc upsmash. This combos at low % and is a decent defensive tool at higher percents as you quickly throw out multiple hitboxes that cover your whole body. Sometimes if I get someone in a pkfire and I want to be stupid I'll run up and magnet jc usmash heh.
Something I'd like to experiment with more is throwing out magnet as close to the ground as possible and wavedashing out of it for a larger variety in followups. Not sure if many would be better than just an aerial but it's worth considering I think.
That's all I got for now.