I recently have been doing similar and have built my own fairly basic JSON parsing library.

Here's a little bit from it:

JSON Typealiases

typealias JSON = AnyObject
typealias JSONObject = [String : AnyObject]
typealias JSONArray = [JSON]

JSONDecodable Protocol

// Represents an object that can be imported via JSON
protocol JSONDecodable
{
    func decodeJSON(json: JSONObject)
}

JSON Type Parsing

private let iso8601DateFormatter : NSDateFormatter =
{
    let dateFormatter = NSDateFormatter()

    dateFormatter.locale = NSLocale(localeIdentifier: "en_US_POSIX")
    dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"

    return dateFormatter
}()

func jsonString(object: AnyObject?) -> String?
{
    return object as? String
}

func jsonBoolean(object: AnyObject?) -> Bool?
{
    return object as? Bool
}

func jsonInt(object: AnyObject?) -> Int?
{
    return object as? Int
}

func jsonFloat(object: AnyObject?) -> Float?
{
    return object as? Float
}

func jsonDate(object: AnyObject?) -> NSDate?
{
    switch jsonString(object)
    {
    case .Some(let dateString): return iso8601DateFormatter.dateFromString(dateString)
    case .None: return nil
    }
}

func jsonDateString(date: NSDate) -> String
{
    return iso8601DateFormatter.stringFromDate(date)
}

Null Coalesce Assignment Operator (a ??= b)

// Null coalesce-assignment operator (a ??= b)
infix operator ??= { associativity left precedence 90 }

func ??= <T>(inout lhs: T, rhs: T?)
{
    // If the right-hand side is not-nil, assign the value to the left-hand
    if let rhs = rhs { lhs = rhs }
}

JSON Result (Enum)

// Completion handler which receives a JSON result
typealias JSONCompletionHandler = (result: JSONResult) -> Void

enum JSONResult
{
    case Object(JSONObject)  // Dictionary
    case Array(JSONArray)    // Array
    case Null                // No Data
    case Error(NSError)      // Error occurred
}

JSON Data Task (NSURLSession Extension)

extension NSURLSession
{
    func jsonDataTaskWithRequest(request: NSURLRequest, completionHandler: JSONCompletionHandler? = nil) -> NSURLSessionDataTask
    {
        return self.dataTaskWithRequest(request) { data, urlResponse, error in

            // Check for network error
            if let error = error
            {
                completionHandler?(result: .Error(error))
                return
            }

            // Check for non-200 HTTP status
            if let httpResponse = urlResponse as? NSHTTPURLResponse
            {
                if httpResponse.statusCode < 200 || httpResponse.statusCode > 299
                {
                    let httpError = NSError(domain: "JSONDataTask", code: httpResponse.statusCode, userInfo: nil)

                    completionHandler?(result: .Error(httpError))
                    return
                }
            }

            var json : JSON?

            // If there is data available, try to deserialize it as JSON
            if data?.length > 0
            {
                // Deserialize the data into a JSON object
                var jsonError : NSError?
                json = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions(0), error: &jsonError)

                if let jsonError = jsonError
                {
                    completionHandler?(result: .Error(jsonError))
                    return
                }
            }

            // Check what kind of JSON object is the root node
            switch json
            {
                // If the root node is a JSON object
            case let jsonObject as JSONObject:
                completionHandler?(result: .Object(jsonObject))

                // If the root node is a JSON array
            case let jsonArray as JSONArray:
                completionHandler?(result: .Array(jsonArray))

                // If there was no JSON data
            default:
                completionHandler?(result: .Null)
            }
        }
    }
}

Example Network Code

let session = NSURLSession.sharedSession()
let request = NSURLRequest(URL: NSURL(string: "http://www.digitlabs.us")!)

let task = session.jsonDataTaskWithRequest(request) { result in 

   switch result
   {
     case Object(let object): // JSON dictionary
     case Array(let array): // JSON array
     case Null: // No Data
     case Error(let error): // Error occurred
   }
}

task.resume()

Example NSManagedObject Class

class Message : NSManagedObject, JSONDecodable
{
    @NSManaged var identifier : String
    @NSManaged var subject : String
    @NSManaged var bodyText : String
    @NSManaged var unread : NSNumber
    @NSManaged var dateSent : NSDate

    var isUnread : Bool
    {
        get { return self.unread as Bool }
        set { self.unread = NSNumber(bool: newValue) }
    }

    func decodeJSON(json: JSONObject)
    {
        self.identifier ??= jsonString(json["id"])
        self.subject ??= jsonString(json["subject"])
        self.bodyText ??= jsonString(json["bodyText"])
        self.isUnread ??= jsonBoolean(json["isUnread"])
        self.dateSent ??= jsonDate(json["dateSent"])
    }
}