My take:
The whole concept of the fast cruiser is to remove 2 "heavy" weapons and replace them with 2 "lighter" phasers, and add more powerful / faster warp drives.
This results in a ship with a great powercurve (often better than the base hull's original powercurve), and the weapons to perform a specific tactic, ie, phaser-boat.
I've noticed the following trends / notes on the various CFs:
The Fed CF and Gorn BF (probably the I-CF, but it doesn't come out till after the Z-refits and practically on top of X-tech) ships all benefit immensely from the conversion. They trade in multi-turn HWs for faster-firing phasers, and the improved powercurve allows them to fly faster / offset that maneuverability problem some.
The Lyran CF (whatever one it is) may benefit, since I've never studied it, I don't know if it loses ESGs for phasers or dizzies. As I don't hear lots of stories / reports of this ship, I would tend to believe that the maulers are better choices for the typical Lyran.
Rommie Fasthawks, I've heard, are a mixed bag. Being phaser-deficient to begin with, losing the heavy-hitting torps to get standard-CA level phaser suites doesn't make it a superior PvP boat, and with the dearth of dedicated Rommies the ship doesn't get a lot of air-time. However, there's usually enough requests / use to show me that there's something there, if you know what you're doing...
Klinks could use their fast cruisers advantageously, I've heard they're up with the Fed / Gorn models. Didn't the Fast-cruiser loving Gorns use them to great effect during the GW series of servers? However, as "traditional" Klinks don't tend to use them, it sounds they fall under the "best limburger is the one nobody smells" effect.
The Hydran CF is another ship I don't hear a lot about. Is it because they don't fly well under "typical" Hydran tactics (ie, they fly faster than their fighters, aren't great at the close-in smack like a fusion boat, and lose the long-range sniping from the hellbores), or are they just not-flied like Klink fast boats are?
Mirak CFs, and I agree with them, were built cheap. They are the lowest BPVed CFs out there, like 20 BPV smaller than the next-smallest CF, and I completely disagree with the weapon choices (they lose drones, have chintzy dizzy arcs, and like Rommies don't really have the phasers to phaser-boat). They do need some study / possible improvement (I'll have to see if the thread I discussed this with Fluf is still on SFC2.Net somewhere).