Deffinetly worth it if you have friends who have it, i have never had so much fun on the xbox, i used to think there was no point of getting online play, seriously get a 3 month as i heard that it may be free soon.
if you can use the etheernet cable, its alot more stable and it is definitely worth it, it works out at about 4 quid a month, not a great deal to ask but it makes the xbox 360 experience complete. basically do get it and use an ethernet cable for better connection
Yes, as others have said use an ethernet cable where possible, it will be at least twice as fast as a typical 54Mbps wireless connection (between console and broadband modem / router) and won't be affected by electrical noise or interference, or when a lorry drives past your house, etc.
Its definately worth it, my only warning being that ive just had someone hack my xbox account so am currently waiting for them to retrieve it for me :( could have been rather costly on my debit card had i not spotted the problem straight away. but other than that ive enjoyed live since the 360 was released massively
Partly depends on age of your son. If he's mature enough to cope with the hundreds of racist / homophobic / xenophobic idiots that appear to live on xbox life (exclusing the sensible mature users of course, of which there are also many) then playing games online (particulary shooters) can significantly extend the life of many games (I've put in well over 240 hours online on Cod 4. VERY sad I know).
If you're at all concerned about who he hears / speaks to I believe there's options within the live account which can be enabled to ensure he cannot communicate with people he doesn't know and they cannot communicate with him. I only use my headset when playing with people I know well as too often I've become irate with ignorant imbeciles online. Thought you should be aware of downsides too though for c£4 per month I personally thinks it's a bargain (and it is VASTLY superior to the PSN on PS3 - I have both so can confirm this).
I use the wireless adapter myself and generally it's pretty good but I can imagine all the comments here re the etehrnet cable are accurate as there's less interference.