I can't speak to the skill levels of the other people in this thread, but coming in as a fairly serious TF2 player who used to play Sniper as main (currently Soldiering full-time):
As stated by various others, the OP is completely wrong on one point - the Sydney Sleeper's damage is identical to that of the original Sniper Rifle. This renders it almost completely useless for a good sniper, because a good Team Fortress 2 Sniper is defined by one thing - the ability to consistently execute quickscope headshots, which deal 150 damage each, cannot be dodged, and have no range-based damage falloff.
Part of the reason the quickscope headshot is prioritized to this level is that the time spent scoped is something on the order of a quarter-second, meaning you suffer almost no hit to your mobility and have very little to fear from spies and other snipers, and dramatically less to fear from anyone else if you're fighting closer to the front lines.
The Sydney Sleeper is completely useless if you do not stay scoped for at least 2 seconds - preferably 4, given that you do negligible direct damage on anything less than a 75% charged bodyshot. This makes you a significantly easier target for attacks by every other class and will either dramatically reduce your longevity relative to simply practicing with the regular rifle, or have almost no effect because the other team have bigger things to worry about. Which of these it winds up being will depend largely on how good your aim is.
Furthermore, a single enemy being afflicted with 8 seconds of minicrits at long range is not THAT significant - the way it modifies damage falloff means that if the player dies, it's fairly probable that he would died anyway, and if he wouldn't have, he still probably would have had to retreat for a significant length of time. He's likely to survive either way if a medic is close enough to prevent him from dying in the absence of the mini-crit effect, so it's essentially a moot point (thrown Jarate is somewhat different here, because it costs almost no time or mobility, can hit multiple enemies and around corners, with at-best medium range, and can put out fires; I'm also led to believe that it lasts longer than that from the Sleeper, but I could be wrong on that point).*
The Sleeper is a good choice exclusively for those snipers who cannot aim reliably enough to be legitimate threats with the original Sniper Rifle, and even then, the difference is never going to be THAT much more than what they'd've observed going for full-charged bodyshots with the original rifle and managing to headshot the occasional Heavy, which has the added bonus of helping them practice with the regular rifle and work towards becoming more useful.
*Bolded because, inevitably, the mini-crits will at some point make a huge difference and enable a team to win entirely because a bad sniper picked something other than the Sniper Rifle. However, my experience thus far has been that the range over which Sleeper snipers can safely charge and fire is not one in which I'm in danger of being killed even with the mini-crits.
EDIT:
I would like to include one point I forgot, and that's that the Sleeper has penetration. This doesn't significantly change my point, because it's pretty rare for people to stay lined up long enough for it to make a difference (and even then, you could get 2-4 quickscope headshots off in the time it takes to charge to full), but it does add another element of separation.
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Now for the opinion point:
I'm honestly not a huge fan of weapons that exist exclusively to make newbie players feel good. This isn't because I'm against newbie players or want them to stop playing - it's because, generally speaking, this weapon will be a permanent crutch and prevent these people from ever actually having to learn how to play at a higher level. Consequently, their fun in the long run will generally be hurt by the weapon's inclusion, because the same good players who were dominating them originally will continue to dominate them until they start making an effort to become good. I recognize that not everyone wants to put in the effort it takes to become good, but given how little fun they'll likely be having if they're playing against people who ARE good, I don't see that it's that much of a choice - if you're going to put yourself through the negative emotions involved with repeatedly getting your ass kicked by someone with more skill at the game, you owe it to yourself to try to reach a similar level of competence so you can reduce the presence of said emotions.
But that might just be me.