Despite the strong reservations of some highly experienced Linux users, Ubuntu is still a good distribution for beginners. Canonical tries their best to make Ubuntu as easy to use as possible. They want it to become a mainstream desktop operating system; therefore a polished user experience is high priority.
Your question about the "strength" of Ubuntu is somewhat ambiguous. I don't think it can be answered without a healthy dose of bias - especially considering the strong stigma in the open-source community surrounding Canonical and Ubuntu - but I will do my best to give you an objective answer. The way I see it, there are two aspects to Ubuntu's strength: technical and political.
The technical aspect of a Linux distribution is very important. You mentioned that you were impressed with APT. The Advanced Package Tool is a Debian technology. Ubuntu derives many of its key technical aspects from its base distribution. Debian is a solid community-based Linux distribution with extensive experience packaging software and developing technical infrastructure. Debian has been around for 20 years and is only gaining momentum. From that perspective, Ubuntu's technical base is about as solid as it can get. However, Canonical has been slowly developing their own infrastructure to replace parts of Ubuntu's Debian base. Ubuntu no longer shares an init system with Debian (favoring Canonical's own Upstart over the more standard SysVInit or systemd), the Unity Desktop Environment is famously developed in-house by Canonical, and soon Ubuntu will also be using Canonical's Mir display server in place of Xorg or Wayland. Each Ubuntu release is based on Debian, and therefore inherits most of the technical benefits, but many packages are patched to carry their own Ubuntu-specific code for supporting the software Canonical developed apart from Debian. Politics aside, Canonical has demonstrated a remarkable ability to develop their own infrastructure and maintain their distribution despite the ballooning number of Ubuntu-specific changes made to each release. Therefore Ubuntu seems to be on solid technical footing as of the latest release.
Almost as important as the technical aspect of a distribution is its political environment. Unfortunately Ubuntu has garnered a strong stigma thanks, in part, to Canonical's apparent case of "not invented here" syndrome. While the core Ubuntu developers employed by Canonical have become quite adept at developing the growing number of projects that Canonical has deemed critical to Ubuntu's success, they have also gained much ill-will - intentionally or unintentionally - by frequently clashing with upstreams on projects they do not control and requiring developers to sign a copyright assignment document before their patches are accepted into software Canonical develops. This tension leads to uncertainty in two ways. First, because Canonical owns the copyright to all the code in their projects, they are free to relicense them under proprietary licenses at any point. Most community-driven open-source projects see this as a risk and potential liability. Second, the frequent friction with upstream developers of important components in Ubuntu means that Canonical has to maintain their own patches for many of these projects. This takes away from time the core Ubuntu developers could spend working on other tasks essential to running a distribution. The patched software also has more potential for flaws if it is not modified by developers who are very familiar with the code, further increasing the development burden required to maintain Ubuntu. Once again, Canonical has sees this deviation as essential to their vision, and has arguably handled it well so far.
In short, Ubuntu inherited a strong technical base which its developers have augmented with their own infrastructure. Canonical is committed to maintaining the Ubuntu core and continuing the development of their first-party software. Ubuntu will move forward as a Linux distribution for the foreseeable future. It is up to you to decide whether or not you agree with their ideals and vision of the future.