I just want to side-track the discussion for a bit to talk about Leftovers; Pocket wanted it to be Topic of the Week though no one actually talked about it. (It might be better to include Topic of the Weeks as a separate thread so that it doesn't disrupt the discussion).
Long story short, Leftovers is better when Sitrus Berry if you get 5 turns of recovery from it, worse if you get <4 turns, and the same if you get 4 turns. Therefore, Leftovers is better on Pokemon that usually survive at least 5 turns on the field.
Qualities that are more likely to survive this many turns include:
1) Bulk. This is a given
2) Typing. Bulk by itself mean nothing if you're weak to many types. For example, Terrakion on the basis of stats is a bulky attacker, but this bulk is undermined by its horrid defensive typing (alas it's excellent offensively).
3) Pokemon that abuse turn-stalling strategies. Protect is the obvious example, but it can be amplified with other moves like Substitute. Leech Seed + Protect/Substitute is another strategy. Boosting defensive stats, like with Cosmic Power, also stall for time. Calm Mind and Quiver Dance are also included, but to a lesser extent.
4) Keeping your Pokemon on the field during turns in which it won't be taking much damage, and keeping your Pokemon off the field when it would otherwise take significant damage. This is the most complex quality, but perhaps the most important. For example, Cresselia takes little damage from most attacks and you will commonly find turns with which it takes little damage against both opponents, even if they both double target it. Cress may only be taking about 20% damage per turn, and as long as it continues to remain like this, she will easily make good use of Leftovers. But if you keep Cress in against Volcarona, it can be 2HKOed by Bug Buzz. Leftovers won't help if Cress takes two hits from Bug Buzz, but Sitrus Berry may make Cress avoid the 2HKO. However, this calc is much less relevant if you switch Cress out.
This quality depends not only on the Pokemon itself, but its teammates. For example, it's a lot more risky to switch Cress out of Volcarona's Bug Buzz if your entire team is 2HKOed by it. It's much easier to do so if you have a Heatran on the bench, ready to switch in. Generally, teams with bulkier Mons and greater defensive type synergy create more opportunities for switching that lead to keeping Pokemon on the field during times when they can take less damage. HO teams, however, won't get nearly as many opportunities.
Similarly, if you have teammates that can lessen the damage that your Pokemon takes, Leftovers becomes a better choice. For example, teams utilizing dual screens, and a Ferorthorn on a rain team to protect it from Fire-type attacks (and it still takes Water-type attacks well)
This also extends to the purpose that Cress carries for the team. If your Cress has TR, and you're using a TR team that needs TR active to function, you may be more keen to keep Cress in to take the Bug Buzz to set up TR. If you have a Dusclops waiting on the bench, you may decide to switch out Cress to Dusclops to take the Bug Buzz and then eat a Heat Wave as it sets up TR. If Cress is your only TR user, you have to weigh up the cons between trying to keep your precious Cress fit as long as possible or to set up TR and risk losing her early.
Generally, Pokemon that make good use of Leftovers have a combination of these factors and not just one. For example, Musharna is bulky, but it generally lacks any time-stalling moves (Protect included), and if it's used on a hyper-offensive TR team, it may not get many opportunities to switch out.
To give examples of good Leftover uses, look at my team
. It is a bulky team with great type synergy, so switching out is generally quite easy with this team. Cresselia is very bulky, and I have teammates that can easily take all the attacks that it is weak too; consequently, I found Leftovers better than Sitrus Berry.
My Heatran is bulky, is part of a FWG core, and has Substitute + Protect for time stalling. Cress has Skill Swap and Sunny Day to remove two of its 3 weaknesses, and Gastrodon helps alleviate its Water-type weakness too. Once again, I found Leftovers > Sitrus Berry usually.
My Gastrodon is bulky, has only 1 weakness and can time stall well too with Recover + Stockpile, especially if TR is up. It is a good Leftovers user. However, I'd say Leftovers is only marginally better than Sitrus (on a different team with which switching is harder, I might prefer Sitrus). It is prone to double attacks and receiving heavy damage before it gets KOed, especially considering that it lacks Protect. If it takes a Grass-type attack before I get a Stockpile boost, it probably won't survive.
Finally, it should be said that Leftovers is never always better than Sitrus Berry and vice-versa. For example, I've had some battles in which Gastrodon has survived 10+ turns, but others in which it got KOed after its second turn and Sitrus Berry would have made it live another turn.