Is it OK to make an approach without any straight-in segment at all?
If the aircraft has not established a stabilised approach, a go around is required. According to EUROCONTROL's Skybrary, a stabilised approach is defined as:


  Their Approach-and-landing Accident Reduction (ALAR) Briefing Note 7-1 suggests that "all flights must be stabilised by 1000 feet above airport elevation in IMC and 500 feet above airport elevation in VMC. An approach is stabilised when all of the following criteria are met:
  
  
  The aircraft is on the correct flight path
  Only small changes in heading/pitch are necessary to maintain the correct flight path
  The airspeed is not more than VREF + 20kts indicated speed and not less than VREF
  The aircraft is in the correct landing configuration
  Sink rate is no greater than 1000 feet/minute; if an approach requires a sink rate greater than 1000 feet/minute a special briefing should be conducted
  Power setting is appropriate for the aircraft configuration and is not below the minimum power for the approach as defined by the operating manual
  All briefings and checklists have been conducted
  Specific types of approach are stabilized if they also fulfill the following:
  
  
  ILS approaches must be flown within one dot of the glide-slope and localizer
  a Category II or III approach must be flown within the expanded localizer band
  during a circling approach wings should be level on final when the aircraft reaches 300 feet above airport elevation; and,
  
  Unique approach conditions or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.
  
  
  An approach that becomes unstabilised below 1000 feet above airport elevation in IMC or 500 feet above airport elevation in VMC requires an immediate go-around.


(Source: www.skybrary.com)