expectations and what sort of video you think will be in your future.

As for AVCHD working with Macs and FCE4, the "experience" will depend on the Macintosh you are using. I've used a HF S100 with a 2 year old iMac and using "Log and Capture" it works great.

After using (and continuing to use) miniDV tape I did not find the FCE4 process dealing with decompressing MTS files from the camera at all "tedious". For what it is worth, all editors will need decompressed files in order to deal with them. In this case, your alternative is to copy the MTS files to the Mac and use something else to transcode them (like MPEG StreamClip) - resulting in large file size. But I found using the Log and Capture flow was easy. The HF S21 has a lens filter diameter of 58mm. This means you can add lenses or a DoF adapter then add lenses to it.

I have no experience using any dSLRs which capture video as a "convenience feature". Vimeo shows so nice video from many of them.

Once the video is in the computer, the source of the video has little impact on uploadable formats. FCE has a ton of file types it can render for exporting. I've not hit any "roadblocks".