I used a few of the free sites in the past and they were very hit and miss on delivery. Some get there instantly, others a delayed by hours and some never arrive at all! It's a bit odd when somebody receives a text at 04:00 asking them if they are coming down the pub!
In the end I opened an account with http://www.bulksms.co.uk/ and paid the minimal fee to have messages arrive instantly! I can honestly say that in a few years I don't know of any texts that haven't been received, except ones whereby the recipient has changed number, etc. The good thing is they support delivery reports as well so that youknow they have arrived.