Some extra comments about Securom are in order here. Apart from the restrictions it places on actual usage (internet activation and so on), Securom itself exhibits many properties in common with malware.
1: Games that contain it install it silently and never give any indication that they include it.
2: It stays on your computer even after you have uninstalled all games using it, and there is no official removal procedure for it. It is a little difficult to remove manually and some of its components cannot be deleted with standard Windows programs. (null registry keys, for example)
3: It operates in kernel mode like a driver, which is a possible security hole and creates additional potential for problems. There was an incident with the Mass Effect content update recently, where installing the content set off something in Securom that made systems BSOD immediately upon right clicking anywhere in Windows Explorer. (they did fix it quickly though)
4: It contains a "blacklist" of programs that Sony thinks you're using to tamper with it. If you open them at any time, Securom will detect that in the background and games will refuse to load until you restart your computer. Process Explorer (an enhanced Task Manager) is the most well known example, but there are others as well.
5: It reduces performance in games, particularly loading times but sometimes also the average framerates. There have also been cases where Securom was actually the source of instability in a game and a crack was needed to solve various crashing problems. The rather funny incident a few months ago with Ubisoft issuing a crack as an official patch was a recent example.
6: Some Securom versions are not compatible with certain DVD drives (for the Securom games that require a DVD). Crysis was an example at least when it came out, as it didn't work on a large number of SATA drives. Apparently those are detected as SCSI devices by Windows, and Sony thinks that any SCSI drive must be an emulated drive.

Fortunately, Securom and all of these other systems have been reverse engineered a few years ago and cracks for the games are trivial to find. I always apply a crack before running the exe (for Securom and Starforce, it's the exe that installs the DRM, not the game installer) and never buy a game until I know that fully working cracks are available. Some games will still work fine online with the patched exe and others will not.
Steam is probably less intrusive than this, but still annoying and easy to bypass. I don't know a lot about it since I don't care about most of Valve's games anyway.