Here is the review.
Features / Advantages. 
a.    Insurance eligibility status could be verified for the reasonable no: of Insurances
b.    If the software used is Ims, claims could be straight away uploaded from Ims itself without login into the “Ingenix”.
c.    We could get the instant “Level 1” acceptance report in the Ims itself after uploading the files.
d.    There is an option available for checking the claim status for the reasonable no: of Insurances.
e.    There is an option available for sending the “Referral Request” for the reasonable no: of Insurances.
f.    We can view the “Level 1 & Level 2” acceptance report and the Payer status report in the Message Center.
g.    Payer Lists with corresponding Payer Id’s could be instantly downloaded from the online itself.
h.    Existing connection of ERA Payer List could be downloaded instantly from the online itself.
i.    The EMC agreements could be downloaded from the online itself.
j.    With the help of highly integrated “Search for provider reports” tool, one can examine and ensure the availability of all necessary reports.
k.    By the help of “Manage Provider Information”, one could add/modify the provider related info’ in the separate web page.
l.    Electronic Remittance Advice could be downloaded then and there and viewed on a daily basis.
m.    All Provider related reports viewed in the “Message Center” could be available for the complete one year time period.
n.    With the help of the “Search Engine” which available within the “Message Center”, one could quickly track the reports as per the requirements.
o.    There is a possibility of activating the separate Worker’s Compensation Clearing House in Ingenix.

Drawbacks:

a.    Sometimes rejections cannot be properly identified for the submitted claims.
b.    There might be an additional charges applicable for the paper claim submission.
c.    Unable to send the Secondary Claims thru’ Ingenix.

Personally I feel that Gateway is better than Ingenix.