Hidan IS strong. The fight against Asuma and the Two Tailed Cat proves it. But he's not smart. Frankly, I feel that Hidan's only ability is his immortality + connection combo. He's basically using the God Mode through the Konami Code of Naruto. But that's all he does. He's not fast in terms of fighting (slowest attacker in Akatsuki) or in thinking. Also, he's too arrogant, and that's the reason he got his head knocked off. If it hadn't been for Kakuzu to re-attach his head, he'd have been screwed. And he's too slow to realize that he's knocked out one of Kakuzu's hearts instead of Shika's.
There are too many flaws with Hidan's fighting strategy. It revolves around being able to actually hit the enemy AND drawing blood in the first place. It also requires the creation of the seal. Finally, he needs to be in the seal.
Kakashi is MUCH faster than Asuma. He managed to evade all of Hidan's attacks while dodging Kakuzu's masked shadows at the same time. If it was a one-on-one, I'd say there's a safe chance that Kakashi could run circles around Hidan anytime. Of course, then again, there's the fact that Kakashi knew what Hidan was capable of that time around, and wasn't caught unawares like Asuma.
If we talk in terms of experience in fighting, Kakuzu probably has much more than Hidan, considering that he's survived long enough to fight the first Hokage. And I doubt that Hidan could draw blood from Kakuzu, because Kakuzu has his Iron Body, which makes him virtually invincible against physical attacks. The only reason Kakashi's Lightning Blade managed to pierce him was because of the elemental superiority.
Frankly, I agree that going against the Akatsuki without a plan is stupid. It seems like all of the members have an infinite number of ways to stay alive. Sasori has his puppet switch, Orochi has his body switch every three years, Kakuzu has his threads, Deidara has his(?) clay and Kakuzu and Hidan has his immortality. And while there isn't anything for Zetsu, Itachi, Kisame or Tobi, you can probably bet that they all have ways of coming back. I wish they were easier to kill. >.>
Huh. While writing this, one thing occured to mind. What would happen if Hidan licked his OWN blood and linked himself with himself? Would that kill him? XD