Even though there are lot of site which free SMS , there were some draw back , like they did not have a option of replying back to the same SMS . The Google's SMS have the option of replying back to the same SMS. In Yahoo ! messenger the feature was there , but looks like they stopped long back .
So do you think , the SPAM will be more ? . NO . As Google is offering Gmail customers a credit of 50 SMS every day , the only draw back is , if a person want to send more than 50 , there is no option in gmail , like buying credit etc.... he have to wait for 24 hrs . Its very use full for normal users.
Credit for the bellow to Google :
" You can send SMS messages to your contacts’ mobile phones using Gmail Chat. To do so from Gmail:
Enter your contact’s name in the ‘Search or invite friends’ box in Chat, and select Send SMS from the box of options that appears to the right of your contact’s name. Or, if you already have a Chat window open for this contact, just click Options, and select Send SMS.
In the dialog box, enter a phone number in the ‘Send SMS messages to this number’ field. This feature is only launched in selected countries. Click here for the list of supported countries and operators. If you are not located in one of these countries you can still use it, but you won't see the SMS option in Chat until you enable it manually in the Chat settings page.
A Chat window appears. Just type your message as you would normally. When you hit Enter, the message will be sent to the phone number you entered.
If your contact replies, the text message response will appear as a reply in Chat. These conversations are stored in your Chat history just like regular chats (but keep in mind that you can’t go off the record while communicating via SMS).
SMS charges: Google does not charge for using this service. However, mobile providers' standard charges will apply to any SMS messages sent from a mobile device to Google Chat. Please check your mobile provider's price plan for details.
Note regarding mobile phone subscribers in North America: depending on which mobile plans your contacts in North America have, they may be charged by their mobile providers for receiving text messages.
Note regarding phone subscribers of Pelephone in Israel: your contacts on the Pelephone network will receive your SMS messages for free, and will be charged for replying according to Pelephone's pricelist for core service. Special rates and packages do not apply.
Note regarding mobile phone subscribers in Zambia: your contacts in Zambia will receive SMS messages for free, and will be charged for replying according to their operator's price for sending an international SMS to the USA."