There's little reason to complain when it comes to Riva FLV Encoder. It works quite well within its purview, and it is free. For standard l aspect ratio video, the program will loads and converts many formats just fine. However, wide aspect video and WMV9 are not within this video converter's scope. Most of the video I deal with these days is widescreen.
The Riva FLV Encoder's interface (the program is basically a front-end for the open source FFMPEG, LAME, and FLVtool2 codec tools) is easy enough to learn and use. You can crop and pad, as well as select bit rates, frame rates, and screen size (e.g., 640*480). My only gripe about workflow is the lack of a progress bar. At first, when converting lengthier videos, I thought perhaps the program had locked up--until it returned an "Encoding successful" message and set me at ease. Riva FLV Encoder supports AVI, MPEG-1, MPEG-2, MPEG-4 (XviD, MP4, 3GPP, V3, V1), older Quicktime types, WMV 7 and 8 to FLV files--but again, only in a standard 4:3 aspect ratio.
All in all, Riva FLV Encoder is a much easier way to produce good-looking FLV file than mucking about with the aforementioned interface-challenged codec tools. You certainly can't beat the price. If all you need is normal aspect output, it's a winner.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.