I have been working with Informix as a developer, DBA, and consultant for over 36 years. That alone says something on the pro side. But let me start with the cons since I think that those are few, so let’s get them out of the way.

Con:

• IBM bought Informix database technology from then Informix Corporation in 2001. This is only a con because IBM does not market product, they only market “IBM” brand and concepts like “Smart Cities”, “AI”, “Watson”, etc. Because of that folks stopped hearing about Informix after 2001 and so many think that the product is dead or at least legacy’d. See the Pros on this.
• Because of the above it is sometimes difficult to find trained talent familiar with Informix. (We are out there.)

Pro:

• Informix is easy to program for.
• Since the IBM acquisition 18 years ago, IBM has made more improvements, enhancements, and added more new features to the product than Informix Corp. did during the 18 years of its existence. So joining the IBM product stable hasn’t been all bad.
• Ease of maintenance. There are major corporations that use Informix that manage over 20,000 server instances with fewer than 10 DBAs. That cannot be said of any other RDBMS system to my knowledge. TCO is way below LOW.
• 5–9’s plus uptime designed in, including planned downtime! There are Informix instances that have not been offline for years. There are Informix instances that have had no unplanned downtime and have only been offline for version upgrades and platform migrations for decades! Oh, and an Informix version upgrade requires less than 10 minutes downtime (less than 5 minutes for fixpack upgrades)!
• Informix backward and forward compatibility between releases is legendary. I have clients that built applications for Informix v7.30 & v9.21 in 1989 that continue to run without even a recompilation against the latest Informix v14.10 engine release! Similarly applications compiled against the latest Informix SDK (v4.50) will connect and work correctly against engines as old as v11.10 (circa 2006).
• Extended data type support inherited from Informix’s merger with Illustra (the commercialization of PostgreSQL). Informix supports extended data types as strongly and with performance equivalent to native data types. These include: timeseries, GeoSpatial, JSON & BSON, and others with several new data types promised in the next few releases. Note that in addition to JSON/BSON data types, Informix exposes and supports the entire MongoDB user API and communications protocol permitting any application written for MongoDB to be connected unmodified to Informix.
• Informix also exposes REST and MQTT interface protocols allowing direct calls from web servers to the Informix instance. The MQTT interface allows Informix to be a consumer and repository for MQTT message traffic.
• Oh, almost forgot, Informix’s legendary OLTP performance. In the 1990’s Informix v7.31 was the performance leader. Since IBM acquired Informix every release from 9.30, 10.00, 11.10. 11.50, 11.70, 12.10, and now v14.10 has been faster than its predecessor. Informix 14.10 processes many queries 5–10% faster than v12.10 did. In addition v14.10’s replication secondary servers process transaction log records 10X faster than v12.10 did. The code improvements that enabled the improvements in secondary server latency also resulted in the server recovering from a hard crash 4X faster. Just how fast is Informix? One online sports betting company that uses Informix processed over 2million transactions a second during last year’s World Cup Soccer tournament on a single system. That system was able to ramp up from serving a normal customer load averaging around 700 concurrent users to over 70,000 users in under 30 minutes.