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Currently, complex handlers available by default are,
- ImageHanderImageHandler
- TextHandler
- PatientHandler
- LocationHandler
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However do note that the newly created obs is not tied to the handler class used to create it. For example, an Obs created using the basic PatientHandler can be viewed and edited using PatientFiedGenObsHandler,
and vice versa.
Consider if any of these handlers, or other basic data types such as numerics, text or concepts etc. will meet your requirements. If not, do consider creating and registering a handler class of your own.
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hence the user is displayed an auto complete box where he can start typing patient names.
Below : Screenshot of an Complex Obs created using a concept of type 'Patient'.
Note the inclusion of a link which leads to the Patient Dashboard, letting the user to view the Patient instance easily.
Below : Screenshot of a of an Complex Obs created using a concept of type 'Location'.
Below : Creating complex Obs without using fieldGen handlers. A simple text box is displayed instead of the auto complete 
Below : Obs validation in action 
a Complex domain Obs sans the fieldGen handler. Note the simple text box provided to input the Location id
Below : Obs Validation at work
Below : Complex Obs validation gets tricky if youre not using fieldGen handlers for domain object types 
you're using basic handlers
Please note that both complex concepts and complex obs can be voided, edited and un voided.