Mission, Mission, Mission. 
I've been an owner since 2006. A cub is great for dicking around low or landing on riverbanks if that is your thing - but as others indicated - a few things:
1.  MONEY. Are both you and your partner financially able to afford it? 
2.  Depreciation:  Hate to break it - but $120,000 depreciates 20% in the first year. Find a used bird where it has a good MX history and someone already ate the depreciation.
3.  MISSION - what is your gameplan? Do you plan to be bumping around the mountains in and out of 2000 ft strips with no STOL or Turbo? 
4.  Join an owners group - see what their typical threads are on MX, operations and headaches. Not everyone will post, but you'll notice things that are nuisances in type clubs (i.e. worn turbo brackets, corrosion areas, etc)
5.  A partnership is a marriage (yes, no toejam). Have an LLC or some type of seperate entity created where the aircraft will reside so your assets will not be at risk incase of an accident and lawsuit. This won't necessarly prevent you from being sued for some roadkill claim, but if you have your liability limits set high enough, it won't matter. Have a clear exit strategy from the partnership if your mission requirements diverge from your partners. *i.e. if A wants out, you sell at loss and divide the difference, or he buys you out at X - the prorata engine hour.
6.  Calculate the per hour reserve. Its not just gas and oil. I'll give you an idea of how mine looks on a per hour basis.
      Cirrus SR-20
Assumptions:      
-   -   
Fuel burn @ cruise      9.5
Fuel cost      $3.50
Oil burn @ cruise      0.08
Oil cost      $7.65
Filter cost      $20.00
Annual inspection      $3,000.00
Static sys./xpdr checks      $500.00
Oil change      $350.00
Engine life      2000
Engine OH cost      $37,000.00
Prop life      2400
Prop OH cost      $2,000.00
Registration, state      $400.00
Insurance      $1,660.00
Hangar/tiedown      $100.00
Preventive Maintenance      $4,000.00
      
Paint      $12,000.00
Interior      $8,000.00
      
Tire cost *3      $360.00
Tire life      800
      
Turbo life      1000000
Turbo OH cost      $0.01
Top-end life      2000
Top-end OH cost      $0.01
Sparkplug life      500
Sparkplug cost      $258.24
Vacuum pump life      100000
Vacuum pump cost      $0.00
   Direct =   $75.08   /hr.
   Indirect =   $8,176.67   /yr.
And, the total annual cost for flying is         $25,445   /yr based on 20 hours per month 
And the DOC broken down as performance numbers for my aircraft:
      Cirrus SR-20
      (OperateLOP)
Engine HP, max.      200
Fuel, usable      52
Fuel consumption @ 75%      10
      0.39
Endurance 75% - no reserve      5.2
Speed @ 75%      140
Distance, max.      728.0
      
Cruise  Performance      
  Power, cruise      70%
  Speed, cruise      137
  Fuel consumption, cruise      9.3
  Endurance, cruise      5.6
  Distance, cruise      762.3
  Fuel economy      14.7
      
  Trip distance      1200
  Trip time      8.8
  Trip cost      $970.29
  Passengers + crew      4
  Aircraft operating cost      $110.63
      $0.81
  Trip cost/person      $242.57
      
Gross weight      3000
Basic empty wgt      2083
Useful load      940
Fuel, max.      52
Payload      628
      
Number of partners      1
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These are big numbers. If you aren't mentally prepared for big numbers don't do it.