Retrieve an Element's Value

var status = xDoc
    .Descendants("optinStatus")    // get the optinStatus element
    .Single()                      // we're expecting a single result
    .Value;                        // get the XElement's value

Example

Here is a working Fiddle for you. You can see it running live here. The output is 3.

using System;
using System.Linq;
using System.Xml;
using System.Xml.Linq;

public class Program
{
    public static void Main()
    {
        var xDoc = XDocument.Parse(xmlString);
        var status = xDoc.Descendants("optinStatus").Single();
        Console.WriteLine(status.Value);
    }

    private static string xmlString = @"

<response op=""getcustomerinfo"" status=""200"" message=""ok"" version=""1.0"">
  <customer>
    <totalMsg>3</totalMsg>
    <custId>9008281</custId>
    <custName></custName>
    <timestamp>2015-04-30 16:17:19</timestamp>
    <optinStatus>3</optinStatus>
    <custMobile>6185312349</custMobile>
    <subacct>1st Choice Courier</subacct>
  </customer>
</response> 

    ";
}

Explanation

Descendents() is an instance axes method (or just axes in shorthand). It returns an IEnumerable<XElement> of all matching descendents. On its results, we call Single(). It is a Linq method that returns the only element of a sequence. If there is more than one element, it throws an error. We're left with a single XElement. This represent an entire XML element. Since we only want its value not the entire element, we call the Value property. Bingo, we're done.

A Bit More Detail

Axes come in two kinds:

• Instance axes methods, which MSDN lists here, are invokable members of the XElement, XDocument, and XNode classes.
• Extension axes methods, which MSDN lists here, are invokable on collections.

With one exception, an axes method returns a collection of type IEnumerable<T>. The exception is Element(), which returns the first matching child object. That what AmatuerDev used and, as in your question, if you are only expecting a single result, it is a just as good if not better approach that is Descendants().

Retrieve an Attribute Value

Once we have an XElement, we can retrieve one of its attributes instead of its value. We do that by calling the Attributes() method. It returns the matching XAttribute. Since we only want the attribute value, we call the Value property. Voila.

// for attribute
var response = xDoc.Descendants("response").Single();
var attr = response.Attribute("status");

General Approach

Using Linq to XML is a two step process.

1. Call an axes method to obtain an IEnumerable<T> result.
2. Use Linq to query that.

See Also

Here is some relevant MSDN documentation:

• How to: Retrieve the Value of an Element (LINQ to XML)
• LINQ to XML Axes