According to Wikipedia:
The AirTunes part of the AirPlay protocol stack uses UDP for streaming
audio and is based on the RTSP network control protocol.[12] The
streams are encrypted with AES, requiring the receiver to have access
to the appropriate private key to decrypt the streams.[13] The AirPort
Express' streaming media capabilities use Apple's Remote Audio Output
Protocol (RAOP), a proprietary variant of RTSP/RTP. Using
WDS-bridging,[14] the AirPort Express can allow AirPlay functionality
(as well as Internet access, file and print sharing, etc.) across a
larger distance in a mixed environment of wired and up to 10 wireless
clients.
See the page here.
Notice the part where it says that UDP is used for streaming audio.
UDP does no error correction. When using TCP (most of the time), when I send a packet from A to B, I send it, I get confirmation, then I send the next packet. When I get confirmation, part of that confirmation is an error check code. If it doesn't match with the error check code that I generated when I first sent the packet, I know the data has been malformed in some way before reaching B, and therefore I send it again.
If I have to send that packet a bunch of times before it is successfully read on the other side, that would "sound like" something unexpected. Lag is a bit different -- lag happens when there's a delay of some sort, or the line is saturated and it actually takes (some arbitrary amount of) time for the signal to get there. That could happen under either model.
The difference is, with UDP, if packets get lost or corrupt, it doesn't matter. iTunes will send whatever data is required to represent the song now. And now, and now. If you miss it or it gets malformed, oh well. It will keep sending only what is pertinent now.
Long story short: you should be fine. Any glitches will probably be short and sweet. And, out of sync? Probably not discernible to any human.
Best Answer
Honestly I'm not aware of any (free) options that I can vouch for. I've used a couple of solutions with little success. What I can offer, however, is some praise for the AirFoil solution which has served me well over the past couple of years.
In terms of VLC, I would also add that there is an issue with syncing the audio to the video when using AirPort and VLC (or any other video player). The Rogue Amoeba folks get around this by supplying their own video player, the Airfoil Video Player, which is capable of handling most video inputs and also making sure to keep the video display in sync with the audio.
So, while this doesn't answer your question as to other options, hopefully what I can do is allay your fears about wasting $25 on a product that may or may not work. In my experience it works exactly as advertised. And I've had good luck getting responses from the developers when I've had questions or minor issues.
Highly recommended software.
Good luck in your search.
-Steve