Strengths and weaknesses

Strengths of ultrasound imaging
It images muscle and soft tissue very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces.
It renders "live" images, where the operator can dynamically select the most useful section for diagnosing and documenting changes, often enabling rapid diagnoses.
It shows the structure as well as some aspects of the function of organs.
It has no known long-term side effects and rarely causes any discomfort to the patient.
Equipment is widely available and comparatively flexible.
Small, easily carried scanners are available; examinations can be performed at the bedside.
Relatively inexpensive compared to other modes of investigation (e.g. DEXA, computed X-ray tomography or magnetic resonance imaging).

Weaknesses of ultrasound imaging
Classical ultrasound devices have trouble penetrating bone but current research on ultrasound bone imaging will make it possible with dedicated devices in the future.
Ultrasound performs very poorly when there is a gas between the scan head and the organ of interest, due to the extreme differences in acoustical impedence. For example, overlying gas in the gastrointestinal tract often makes ultrasound scanning of the pancreas difficult, and lung imaging is not possible (apart from demarcating pleural effusions).
Even in the absence of bone or air, the depth penetration of ultrasound is limited, making it difficult to image structures that are far removed from the body surface, especially in obese patients.
The method is operator-dependent. A high level of skill and experience is needed to acquire good-quality images and make accurate diagnoses.