AFs can be fun in PvP when you're in a good sized gang.
You can breeze through lvl 1 & 2 missions and you can use them for lvl 3's if you just want the challenge, but they do 3's very slowly because you have to hit/fade all the time. AFs in lvl 4 = dead AF, you just can't tank the dmg and you don't really have the DPS to kill the higher bounty ships.
Now if you meant to say HACs (heavy assault cruisers) then yes, some are VERY devastating in missions and others are just useless. Basically, any HAC that relies on tanking and dmg output works great. Any HAC that relies on EW and/or speed instead of a tank will get chewed up.
Basically, assault frigs are special case ships that are great for large PvP gangs. I've used the Hawk, Ishkur and Enyo and am training for the Amarr AFs currently. So far my favorite is the Ishkur because it's basically a mini-Ishtar. Enyo was fun but not as usefull. Hawk has impressive alpha dmg for a frig but the overall DPS is dissapointing, plus it's slow no matter how you fit it. I have high hopes for the Retribution
If you want to get into small ship warfare though, the best way to do it is first realize you're training to be support for a larger group. You want to get Interceptors out of the way first, because frankly they're always useful no matter how many you have in the gang.
Next, look into cov-ops ships and see how long you have to plan for stealth bombers. The most fun I've had in EvE is in stealth bombers, but ONLY train for the Manticore or the Nemesis. The Minmatar and Amarr SBs are garbage.
All the AFs have their strong points and weak points, they're all basically mini-tanks with above average DPS (for a frigate) except the Ishkur which has amazing drone capabilities, and the Jaguar which can reliably "speed tank" like an Interceptor. Just remember AFs are support ships for larger ships that can't kill interceptors and drones, you're not going to kill a battleship in your Enyo.
Finally, there's the EW frigs. I'm training to use the Sentinel which is a mini-Curse and sounds like a lot of fun. The Caldari Kitsune is a REAL bitch from what I hear, with its ability to lock down battleships and basically remove them from the fight indefinitely. The Hyena is a fast webber which woudl be useful for roaming gangs and possibly gatecamps, but not larger fleet battles IMO. At a glance the Keres doesn't appear to be any better than it's tech 1 sibling for Damping, so I wouldn't advise training up for that one unless you just love Galente.
Again, these are support ships for a larger gang and not powerful enough to take on large ships solo, however a well balanced gang/fleet of nothing but tech2 Frigs can be VERY nasty against a fleet comprised of mostly large, slow moving/targeting/tracking Battleships.
If you want something to blow through missions and make money, train for battlecruisers/battleships. If you want to PvP and be the tank/dmg dealer, train for tech 2 cruisers and battlecruisers.
If you want to PvP cheaply, know you can get a gang on a regular basis, and aren't the kind of person that has a shit-fit from dying in a video game, train tech 2 frigs, all of them. And don't forget to train up the corresponding skills to the ship you want to fly, the bare minimum requirements to fly the ship are not enough.