Considering a First Watt F-4

Looking to get a F-4. Researched, I have a high gain pre-amp, however not much in the way of reviews. Any opinions, experiences with a First Watt F-4?
I just got a J2 so cannot comment about the F4, but I am extremely happy with the amp. I am amazed at how nice a 25 wpc amp sounds and the build quality is what you would expect but from Mr. Pass but at a bargain basement price which makes it a low risk experiment, which is watt this hobby is all about and that is what makes it fun and expensive.
You're not getting deluged with replies, so I thought I'd chime in.

I also own a J2, in my case driving my horns. It's everything Photonman said. If you buy from Mark at Reno Hi-Fi (as I did), as far as I understand things they will let you try out the F4 for a certain period, and return it if you're unhappy.

I wanted to make sure you understood that the F4 is a buffer, and has no voltage gain, so it requires a fairly hefty voltage drive to reach full power.

I owned a Rawson F4 clone. Sound quality was outstanding, but it just didn't work very well with my Zu Druids. With the F4, it's difficult to separate power issues from gain issues. My preamp had 20dB gain and I suspect that wasn't enough, but it's also possible the F4 didn't have enough power. If your preamp's gain is higher than 20 dB and you have relatively high efficiency speakers, it may work great for you.