Roserade is a favorite Pokemon of my sister, for understandable reasons. It has good stats in the right places, an okay typing, a good movepool, and a cool design. It sadly lacks that "oomph" factor needed to bump it up to OU, but in UU it can really shine.
Recommended tier: UU
Poison Point is a pretty good ability in-game, but it's pretty useless competitively. A Pokemon that unleashes an attack that makes direct contact with Roserade has a 30% chance of being poisoned. Since regular poison can't compare to burn or Toxic poison, this is pretty unfavorable.
Natural Cure is one of the best abilities in the game. Any status afflictions on Roserade are healed upon switching out. This is especially favorable against Sleep and Paralysis, which sweepers hate.
Technician is Roserade's Hidden Ability. It multiplies the power of any move that has a power of base 60 or less (prior to STAB and possibly other effects) by 50%. Sadly, Roserade only has a couple of moves that benefit from this ability, but at least its not completely worthless.
Roserade's stats definitely put it in the category of Special Sweeper. It's a lot faster than most other Grass types(unless Chlorophyll comes into effect), and its base 125 Sp. Attack is the highest of any Grass type. Its Sp. Defense is also good at base 105, but base 60 HP and base 55 Defense means being defensive is out of the question.
Roserade's movepool possesses a good mix of offensive and support moves. Leaf Storm is the most prominent of its STAB attacks, with a whopping base 140 power. Of course, that nasty Sp. Attack drop is unfavorable, but Roserade usually doesn't outright sweep anyways. Petal Dance is another option, acting like a special Grass type Outrage thanks to the big power boost it got in the generation shift. Energy Ball and Giga Drain are weaker, but lack the nasty side-effects. Magical Leaf can be substituted on a Technician Roserade, as it gets 90 base power prior to STAB with the boost. Sludge Bomb is another STAB option, but its poor coverage makes it unfavorable. Shadow Ball, Extrasensory, and Weather Ball are decent fillers. Most Roserade will also want Hidden Power Ice or Hidden Power Fire for coverage.
Roserade's Attack is decent at base 70 and can be improved with Swords Dance. Unfortunately, Roserade's physical movepool basically consists of Seed Bomb, Poison Jab, and Return, which is a shame. Pin Missile is an option with Technician, but its still weak.
Roserade is a very effective user of Sleep Powder thanks to its good speed. It is tied with Venomoth as the second-fastest user of the move (with Jumpluff being fastest). It can also employ the "double powder" strategy with Sleep Powder and Stun Spore. Roserade can set up Spikes and Toxic Spikes as well, but keep in mind that its frail and Spikes is illegal with Sleep Powder. It also gets Aromatherapy, but its too frail to use the move. Synthesis is in the same bucket. Worry Seed has its uses, and can sometimes force a switch. Roserade can employ the SubSeed strategy as well, although it can't do as well as Sceptile, Whimsicott, or Jumpluff due to its inferior Speed.
Countering Roserade: Most Roserade have Sleep Powder, and many counters can be shut down by it. One option is to sacrifice a Pokemon and activate the Sleep Clause before whaling on Roserade. Steel types generally do well, provided Roserade lacks HP Fire. Chandelure (who we ironically did last week) is one of the best counters, but it too is shut down by Sleep Powder.
Physical attacks are effective, as Roserade has poor Defense. And although Roserade is quite fast, it can still be outrun by a lot of Pokemon and revenge-killed. Priority moves are also effective.
The most dangerous thing about Roserade is its ability to put Pokemon to Sleep. Even so, this flowering plant packs a lot of power. Roserade's Leaf Storm is the strongest one out there; it'll hurt A LOT. This is one rose you shouldn't stop to smell.
My favorite Poison type.