Has the coin really been developed? Has it useful for community with Sia's services?
Yes, Sia is one of the few crypto projects that can brag it has a working product, and actually it has been working for the last 2 years. Currently the storage network is 3.3 Petabytes big with 100TB being used. You can download the client and start uploading your files or offering your some disk space and be paid for that. This chart shows the growth of the used storage network:
Which currency will be accepted during the ICO?
The ICO was over 3 years and half ago... It was not for Siacoins, but for the second asset of the network, the SiaFunds. 1130 were sold at 0.5BTC ($192 at that time). Siacoins were never offered in an ICO: it is a mined coin, and developer team only pre-mined 100 blocks.
Does the team have experience in this sphere?
The co-founder worked for a while on IBM. All of them were MIT students. At this points all of them have experience on the field... because Sia is already a working product
