I'm just going to get this out of the way right now: infect is easily the worst mechanic WoTC has ever made for Magic. Why? Only one card in the entire game actually removes poison counters and that's Leeches (no, Melira, Sylvok Outcast doesn't count because she only prevents you from getting them). Infect basically halves the game time and is incurable so you lose in 2-3 turns no matter what. Even Gisela, Blade of Goldnight has problems dealing with infect because it only takes 10 damage to kill a player. It's just a stupid and not a fun mechanic to play against. From now on, I'm just going to treat cards with infect as only having wither, like it should have been.
Flensermite is actually a pretty good card, but not when compared to other 2 drops of the same nature. Ichorclaw Myr has mini-rampage 2, Juvenile Gloomwidow has reach and is tougher, Rustrazor Butcher has first strike, and Stigma Lasher prevents your opponents from gaining life (which is what infect pretty much already does, but I explained myself already...). At most, it's going to gain you 1 life and weaken something else on its own. Toss a Dark Favor or Unholy Strength on it and it can do a bit more damage, but it's not really worth it. Save for the lifelink, Ichorclaw Myr is a better choice for a black deck to run than this.
Pros: Wither, lifelink
Cons: Weak, dies in almost any combat
Replacements: Ichorclaw Myr