Lazarus versus Delphi

The most obvious reasons for using Lazazus: it is free, and it has a very large user base.

Features that are almost the same

Lazarus uses Free Pascal as its programming language, very compatible with Delphi's Object Pascal.
The Lazarus LCL component library is very similar to Delphi's VCL.
Tons of code written for Delphi can be used with Lazarus, either directly or after automatic conversion.

The size of an executable file is much larger with Lazarus than the Delphi equivalent, because Lazarus stores debug information within the executable rather than as separate files. But the executable file size can be significantly reduced with settings in the Compiler options.
Some file formats: Lazarus uses .LPI, .LPR and .LFM, Delphi uses .DPR and .DFM

The documentation: Delphi's Help system is more complete. The Lazarus Help files that are installed are not very detailed, when you ask for context sensitive help. Most of the Lazarus documentation is found on the web at wiki.freepascal.org.
Tip: you could also download the offline CHM Help for Delphi from Embarcadero for additional info ;)
Delphi's Code Completion is called Identifier Completion under Lazarus.
In Lazarus, Code Completion combines Automatic Class Completion, Local Variable Completion and Event Assignment Completion. All of them are invoked by Ctrl+Shift+C. The IDE determines what is meant by looking at the cursor position.
Lazarus supports Include files, that make it possible to write platform (in)dependent code without messing up your code with IFDEFs.