I used the Base jumper a lot in HL before it was nerfed and it was a fucking dumb item to play against.
unless you are in a tiny enclosed space, any fight with another soldier you have is almost a free win because you surf his rocket into the air and deploy your parachute and now he has an extremely more difficult shot to make to kill you while you're fairly safe in the air and can easily spam multiple rockets from above. This is still possible, but now you're basically a sitting duck in the air and it's a lot easier to get hit.
Even when you were not fighting a soldier, you could easily jump into a flank and deploy it high in the air. From that position, everything that isn't a heavy or sniper is doing less damage to you than you are to them (so nobody on the flank and not any 6s classes) and if you miss your rockets or more people come and the fight doesn't look good, you can very easily just turn around mid air and float away. This part of the Base jump you can no longer do since your air control is so hampered when you have it out now.
It basically gave you so much control over any flank that has any decent amount of open space without giving up much and it was REALLY easy.