nopeTino_ -snip-similarly, the xbow isn't the best option 100% of the time either. Sometimes you might want the overdose if you expect to be baited on high ubercharge, or you might just want to run stock syringes for close range sections of maps, eg granary which according to thefragile in one of the myriad xbow focused threads recently suits syringes much better for most of the arena than the xbow.
As for machina, I don't personally like the weapon but it's not a straight upgrade either. There are times where you're gonna want to be able to bodyshot without scoping.
The XBow is the best option outside of 1 scenario that I can think of. An XBow medic will get uber faster and put out a Heals per second than a medic without one which is more than enough reason to run this weapon 24/7, add to it that the effective combat range of medic is significantly increased while also being able to heal people significantly further away then before you should run this weapon 100% of the time. As for that 1 scenario I can think of (that'll make syringe gun relevant) is a last defense where the medic is the only person alive.
Machina script
bind Mouse1 +attack2; +attack; -attack; -attack2
or something very close to that effect.